Distinctive Appearances

This is a concept that transcends almost all forms of media, and even has a related topic even in non-visual forms like radio or literature. When working with a very visual format you do not want people to confuse your characters between each other. You may be capable of distinguishing between your friends but in a story if the random Red Shirt looks very similar to The Hero it can confuse the audience as to why the hero suddenly died with little fanfare. It might seem like a small thing but you do not want to confuse your audience like that.

So in order to make sure the main characters are identifiable you should go out of your way to ensure a distinctive appearance is given for everyone. As a rule of thumb, even though it may seem bad at first, even before you know the person's name other people should know what you're talking about when you talk about "That guy with the Nice Hat" or "His Badass Longcoat was flowing in the Dramatic Wind." If they are that easy to identify from the start then it makes their presence in the story that much easier to follow.

Another popular thing to do is make their shape so distinctive that you don't need color or even see them, just a shadow cast on the wall is enough to know who it is.

The concept is very well known and applied in the professional level, and many examples of this can be quoted directly off the DVD special features.

And this can go in almost any number of directions.
 * You have a Five Man Band, a group who is defined by their roles and quite often their physical size and appearance. You want these characters to be memorable, so you make them as different from each other in appearance as possible. The Hero is muscular, The Lancer is slender, The Big Guy is really huge, The Smart Guy is really small and oversized glasses, The Chick has all the right curves, etc.
 * Have it all be Color Coded for Your Convenience. Power Rangers has survived on this for years, the Red Ranger is the leader. The Pink Ranger or Yellow Ranger is the chick. Not bad for a team costume that covers their face, huh?
 * Make it a Five Token Band, even if all you know about the guy is that he's "The Mexican one" that's a lot better then "the other white guy."
 * Clothes Make the Legend
 * Dress Coded for Your Convenience

And compare/contrast any number of tropes, including Cast of Snowflakes, Only Six Faces, Palette Swap, Limited Wardrobe, One Steve Limit.

Anime

 * Cowboy Bebop made the main cast all very distinctive, with the slender Spike, muscular and bearded Jet, curvy Faye and waif redhead Ed. Apparently the designer even said that he avoids drawing characters with perfectly symmetrical appearances and that is why Jet had the mechanical right arm and the metal piece under his left eye.
 * This is probably one reason why the main character in a Gundam show always pilots a Gundam - to stand out from the crowd (and sell more model kits). Only 08th MS Team subverts this by giving Shiro the same MS as his comrades... until it's badly damaged and he gets a customized unit.
 * It is hard, very hard to mistake one character for another in Tower of God. It could be the fact that one of the main characters is a miniature version of Godzilla. Maybe because that Purple Jumpsuit only belongs to one person. Anyway, despite having very similar faces (except for Godzilla and the other Lizard Folk) character designs tend to so one-of-a-kind that most characters can be identified from afar. That even goes for all the background characters from season one.
 * Edward Elric's bright red coat. Most of the other State Alchemists wear the same uniform but are visually distinct enough that you can tell them apart, in part due to varying body types.
 * Only in the original anime, though. While he does wear it in the manga and Brotherhood, its appearances become much less frequent as the story goes on.
 * Isumi of Hayate the Combat Butler doesn't wear the standard girl's uniform at their school. This is even lampshaded when Nagi invites her to come see the new kitten: she says she needs to go home first and change out of her school uniform and Nagi responds that she's wearing a kimono already. The Hakuou girl's uniform is already similar to a kimono, but Isumi says it's too breezy.
 * Oda, the creator of One Piece, goes out of his way to make sure every non-mook has an appearance distinct from everyone else (except for a pair of Single Minded Twins).
 * Sailor Moon has the title character wearing her trademark Odango Hair with long twintails, making her very distinct and recognizable throughout the whole Magical Girl genre.

Comic Books

 * Batman's silhouette is as easily identified as his appearance, most of the time it is the same thing as his insignia. Part of it is intentional, when his body and cape casts a shadow even the mooks know he is there.
 * Wolverine with his claws extended in the crouch stance.
 * This was Todd Mc Farlane's objective when he started drawing Spider-Man, and it mostly worked. His silhouette is pretty standard... except that it's often contorted into odd poses, and sometimes those big white eyes are staring out of it.
 * In early Superhero comics, colorful costumes and iconography where often the only way to tell heroes apart due to level of art (and quality of the color printing) produced at the time. This is why Batman and Superman were able to pass as each other when needed for the longest time.
 * This trope can be difficult to pull off well in war comics. Every character generally must be a young male, wears the same uniform (usually including a helmet or other headgear), often necessarily of the same ethnic background, etc. Giving characters distinctive appearances without violating basic military realities presents a lot of challenges - particularly if the artist already has Only Six Faces tendencies. Joe Kubert's character designs for Sgt. Rock's Easy Company are a good example of an artist successfully giving soldiers Distinctive Appearances despite all those obstacles.
 * Played for Laughs in the Canadian Stevie Diamond book series by Linda Bailey. The titular character has distinctive frizzy hair, and she becomes incredibly self-conscious about it when the lifeguard at the local pool tells he to get out of the pool by yelling "Hey You! The kid with the hair!" That' right, the hair.

Fan Fiction

 * Anyone who has tried to illustrate My Immortal has run into this problem. Everyone is deathly pale. Everyone has dyed their hair black. Everyone wears the same "goffic" clothes. All the distinctive aspects of any character's canon appearance (Harry's glasses, for example) are soundly eliminated.

Film

 * Indiana Jones and his Nice Hat. He walks into a room and you know it is him. You see his shadow and you know it is him, and just to be on the safe side he carries a whip on his belt in case of other hat-wearing characters. You can see him in a crowd and know which one he is on a bridge. So much so that the filmmakers eventually decided that his hat would only come off if there was a darn good reason. Note the footage of Harrison Ford jokingly offering to staple the hat to his head.
 * Van Helsing of...Van Helsing has a similar Nice Hat for similar reasons, as Stephen Sommers has said: makes it very easy to tell who he is, even in a long shot.
 * Several monsters created by Toho certainly apply here. One can easily recognize Godzilla, King Ghidorah, Anguirus, and Gigan for example due to their distinctive appearances.
 * Star Wars: Is there anyone who can't
 * The lightsaber colors were meant to be easily distinctive between Jedi and Sith. And in a literal vein, Luke's new lightsaber in Return of the Jedi was supposed to be blue, but with most of the Sarlacc pit battle being outside against the blue sky they changed it to green to stand out.
 * Also, apparently the reason Samuel L Jackson wanted a purple lightsaber was so he could pick himself out in crowd shots.
 * During production of The Incredibles, an animator did outlines of each of the Parr's heads, to make sure each of them was distinct from the other. Brad Bird liked the sketch so much he had it made into a T-shirt.
 * Yep. Not only color, but also silhouette, pose, and light-to-dark ratio/location.
 * This was talked about on the DVD of Spider Man 2, where they said it was really fun to create Doctor Octopus because all you needed was his silhouette and you can tell it is him.
 * In Rush Hour 2 the fight in the massage parlor had all of the mooks wearing blue robes with Jackie Chan wearing a purple robe and, well, Chris Tucker stands out with little trouble, but they still had him do the fight shirtless.
 * In an aversion, the Transformers films have been criticized because the robot characters look too much alike, especially in fast-paced fight scenes. YMMV.
 * In The Triumph of Sherlock Holmes, Balding is a good head shorter and a good foot rounder than Murdoch. This makes a major plot point pretty perplexing.
 * This was an issue in designing the Reliant for Star Trek II the Wrath of Khan. Since the Reliant is also a Federation starship, it had to look like it came from the same fleet as the Enterprise. At the same time, however, the Reliant is hijacked by the villains and is their ship for most of the movie, so there was a concern that audiences would become confused if the "good ship" and the "bad ship" looked too much alike.

Literature

 * Subverted in the Knight and Rogue Series. Fisk and Michael are both plain looking. So plain looking that people trying to catch them have an easier time trying to ask about their horses. However, Michael happens to have a scar on his face that, in the second book, is faked by a framer so that everyone will be on the lookout for that particular distinguishing feature.

Live Action TV

 * Scrubs had all the people in the hospital separated into colors: white lab coats or blue scrubs for medical doctors, green scrubs for surgeons, pink (or purple) scrubs for nurses and the maintenance crew wear black. Even then J.D. usually wears a different shade of blue than most of the other doctors (or at least Elliot, who is also a medical doctor). Dr. Cox and Dr. Kelso are generally the only main characters to regularly wear labcoats.
 * Individual characters usually do something to look unique. The Todd always has his sleeves ripped off to show his "guns." Doug wears a much lighter shade of blue along with a red fanny-pack.
 * You can also look at the ethnicity and general size of the characters. J.D. is very skinny and dark hair, Turk is black, Carla is Dominican, Dr. Cox has curly hair and is muscular, Elliot has Hair of Gold, Dr. Kelso is short and heavy set, and The Janitor is tall and looming.
 * In the episode where J.D., Turk and Carla all got scooters, they matched the colour-coding of their scrubs, in what might have been a mild Lampshade Hanging.
 * Star Trek did an iconic color coding of their uniforms, along with different insignias and patches that indicate which department they are with. TOS had Operations wear gold (actually a lime green, like Kirk's wraparound tunic, but the lighting on the standard uniform fabric made them look gold), Engineering and Security wore red and Science and Medical wore blue (McCoy even had a short sleeve tunic to make it clear he was a doctor). Starting in TNG, the rank was designated by pips on their collar and the department colors changed: Operations wore Red, Engineering and Security wore Gold and Medical and Science wore blue as always (Dr. Crusher often had a blue lab coat).
 * When Star Trek Deep Space Nine came out they introduced new uniforms to have colored shoulders rather than colored torso's, originally made to differentiate Starship crews from Starbase crews. In Star Trek Generations the colored shoulders uniform was given to the command officers on the Enterprise, making it the only time both variations were worn by the same crew.
 * They also introduced a special captain's uniform to make them different from other operations staff. Kirk had the green wraparound tunic (acknowledged with Enterprise's Mirror Universe episode). Picard had a two layer outfit with a velvet jacket over a gray shirt. Star Trek First Contact introduced the Starfleet uniform change with a black and gray jumpsuit with color coded turtleneck undershirts, but Picard spent most of the film wearing a red shirt under a vest. That same outfit was later worn by Sisko in Star Trek Deep Space Nine.
 * Admiral Uniforms also tended to be different, often with more insignias and a front zipper. Star Trek Enterprise surprised fans by having the Admiral uniforms with a necktie.
 * Green Wing does something very similar to Scrubs, at least to a certain extent, which obviously makes sense when they're both set in hospitals. Oddly, while all three wear green scrubs in the operating theatre, Mac will continue to wear his while Caroline and Guy change back into their white lab coats.

Newspaper Comics

 * This is why the main Peanuts characters have different patterns on their clothing. Charlie Brown has the distinctive zigzag, Linus wears stripes, Sally has polka dots. When Charles Schulz realized that Rerun looked too much like Linus, he gave Rerun a flatter hairstyle and ever-present overalls.

Video Games

 * Gears of War designed The Hero Marcus as having the default armor style for the game, with everything else being derived from his design. For Dominic they gave him a shoulder light as well as a knife on his chest to give him a different silhouette. Cole has bare arms and Baird has blonde hair with blue armor.
 * Valve deliberately invoked this when designing the characters in Team Fortress 2. Every class has a very distinct shape which helps a lot to identify what's coming after you. This also use to make it easy to spot snipers by looking for the hat, until just about everyone gained customized hats that can throw this off.
 * Also by Valve, the characters in Left 4 Dead all have distinctive silhouettes as well as the various special infected.
 * Mass Effect 2 gives Legion a massive hole through his chest, visible from almost all angles, so the player can distinguish him from his hostile cousins at a glance and avoid friendly fire incidents.
 * Legion also has welded to his chest.
 * Despite being very much a Improbably Female Cast, it's possible to tell nearly every member of the Touhou cast apart by their silhouette, as demonstrated in this video.
 * Every single Robot Master in the Mega Man series is designed with a distinct body, which helps a lot when you consider how they tend to have similar faces. Many can also be told apart by their silhouettes (much like the Touhou example above), if 9 and 10's promo videos are any indication.

Web Comics & Web Originals

 * Due to their unique accessories, and the careful design of their fur and tails, no two characters in Bittersweet Candy Bowl have the same silhouette
 * One chapter in How to make Webcomics (By Kurtz, Straub, Guigar, and Kellet) even went so far as to describe the (black-and-white) layouts of the different characters in PVP, pointing out the different areas as light and dark, and how no two characters had the same arrangement.

Western Animation

 * Transformers does this most especially when the toy designs were actually for the franchise and not repackaged from other toy lines. There are still some things you don't mess around with, such as Optimus Prime's chest windows. In fact, since most of the other Autobots have the front grill of the car become their chest, having the front windows become his chest is what makes him visually unique and thereby the leader. With Transformers Animated, the design is so unique that all you need is their outline and you can see the Autobot line-up.
 * It should be noted that the "law" of Prime design has been averted on three occasions: once, in Armada, where he was a long nosed cab and instead his grill formed his chest, and then later in his Alternators and Alternity incarnations, in which he was portrayed as having a chest formed from the front end of a truck and car, respectively.
 * Gargoyles come in all shapes and sizes. None of the main crew have the same color scheme, same wing design, or even the same face. In fact, seeing that Angela had Goliath's coloring but Demona's appearance tipped Elisa off that she was Goliath's daughter.
 * The original character design for The Simpsons required the family to each have distinctive silhouettes (only Maggie and Lisa have the same hair). Most other characters could probably be recognised by their silhouette too.
 * It's actually one of creator Matt Groening's rules that the character design department MUST make all new characters instantly recognizable by silhouette, but not draw attention from the main family, or it gets rejected.
 * This can also be seen in Futurama where Matt Groening described Fry's "hair horns" as being particularly useful in maintaining a unique silhouette.
 * The Phineas and Ferb creators used to work on The Simpsons, so they designed the major characters based on the same concept. The title characters are the most obvious examples, especially Phineas, who has a triangular head.
 * Adult Swim had a between-shows promo that mentioned this, with silhouettes of the cast of Futurama clearly identifiable.
 * Even if Henry and June from Ka Blam! wore the same outfit, had the same eyes, and sported the same hair, there's a way to tell them apart. Henry has freckles, and June has some baby fat she didn't burn off yet.