Undeathly Pallor

In fiction, if you run into someone who looks like a corpse it's not that they're sick, it's that they're dead.

Characters who become one of The Undead will develop pale or sometimes blue or green tinged skin. In severe cases with visible (and deathly still) purple veins underneath the skin. Even people of color who were dark skinned in life will have it turn ash grey, or dark like soot.

A living character who is just pale is usually a perfectly mundane case of an Eerie Pale Skinned Brunette, who has heard all the jokes about waking up on the wrong side of the coffin. There may be some Unfortunate Implications here though, since in fiction there's a strong correlation between being pale and going over to The Dark Side. In addition living characters who come into contact with the dead, undead, or infernal may have a change of color and become Hades Shaded. This is the case with necromancers, infernalists, those under ghostly Demonic Possession, someone bitten by zombies or a vampire, or The Renfield who has fed on vampire blood. It doesn't stop there either, centuries of sunless existence may turn the character into an Evil Albino.

This trope is so common it's usually a (pun intended) dead giveaway that something is wrong with Alice / Bob when they suddenly show up this shade of pale. Smart characters (let alone the Genre Savvy) will have their Not a Zombie sense ping them as being "somehow off". Which of course makes the aversion of this trope a #1 priority on every smart undead's list of Masquerade reinforcing tricks. Settings where this trope is subverted have much more dangerous undead for being indistinguishable from the living. Another curious exception is that sometimes people who were brown-skinned or darker in life may look exactly as vivid after death as they did when they were alive for unexplained reasons. Probably something to do with the Special Effects budget and wanting to avoid a gaffe.

Truth in Television, as once you die, the blood moves due to gravity to the lower parts of your body (if you are lying on your back, then it pools there). It's called Post-mortem lividity. So if your skin looks drained of blood, it's because it is.

See also Stringy Haired Ghost Girl.

Anime and Manga

 * Sid from Soul Eater is a zombie gym teacher. He once had normal colored skin, but after dying he became blue... Literally.

Comic Books

 * Dead Girl in X-Statix, who's sort of greenish.
 * Any Black Lantern who didn't just have a skull for a face had grey skin.
 * Malibu Comics' Ultraverse character Ghoul was green (and pretty messy besides).

Film

 * Dark City - The Strangers are
 * Corpse Bride provides the page image here. In this case, however, it's subverted as the living are colorless while the dead have much brighter colors.
 * The alien monster mutants in Pandorum are all Undeathly Pale. It's later revealed to be because they
 * The Morlocks in The Time Machine have a similar reason for their pallor.
 * Twilight plays this trope straight with caucasian vampires, but Laurent presents an odd contrast. He's not at all pale in the first movie, but was washed out and greyish though, at least in the second film.

Literature

 * In the Twilight books, Maria the vampire looked "porcelain" despite being Mexican. The book is pretty inconsistent about what happens to People of Color when they become vampires. Maria was really pale and Laurent was olive-skinned (but it was never stated what race he was). There are some other vampires who are non-Caucasian, but their skin color isn't really mentioned. The male half vampire from South America was definitely dark skinned, though.
 * The Twilight Illustrated Guide states that all vampires turn pale on being turned. If you were black in life, you turn olive. And since vampires are always beautiful...
 * Variation in the Old Kingdom series. The actual Dead aren't noted to be pale (since most of them that actually have phyiscal bodies are too badly decayed to tell) but Necromancers (as well as Abhorsens, who practice necromancy to destroy, rather than create or control, the undead) are described as noticeably paler than those who don't practice that art. So it's pallor from associating with death and the undead, rather than from being dead yourself.
 * In Warbreaker, when a Lifeless is created, all color is bleached from their body as a side-effect of the magic (which uses color as a sort of trigger). The resulting creatures look exactly like they did in life, except that they are pale grey all over.

Tabletop Games

 * The Abyssal Exalted get this in spades - As they get more powerful, Abyssals are required to either become pale and beautiful, or decayed and monstrous.
 * In both Vampire: The Masquerade and Vampire: The Requiem, vampires typically grow paler and less lifelike as they grow older -- not because of age, but because an older vampire is more likely to have fallen a few notches down the Humanity scale and gotten closer to their Beast. The one exception may be the Assamites from Vampire: The Masquerade, who invert the trope by growing darker as they age, to the point where the elders of the line look like they're made of polished jet.

Video Games

 * In World of Warcraft the Forsaken have skin in various shades of grey, green and blue. Death Knights also tend to be paler then a living character with the same setting of skin shade. They also have access to a several different skin tones not available to living players, mostly in greens and greys. Though some of them are a bit odd - blood elves apparently decay into charcoal.
 * Brutal Legend has the Drowning Doom, whom all their units have this look to them. Most noticeable is Drowned Ophelia, the doppelganger counterpart of Ironheade's Ophelia, who, naturally, turns from a healthy looking, yet slightly pale Perky Goth, to an openly corpse blue after her transformation into the Queen of Black Tears.

Web Comics

 * In Zombie Ranch people turned into zombies become noticeably green. Other mammals just seem to go green on the inside.
 * The Kingfisher: Pale vampires abound.
 * Being based off of Exalted, Keychain of Creation features various extremely pale Abyssals, the best-known of which is, of course, Secret.

Western Animation

 * Played with in Danny Phantom. Danny's skin when he's a ghost is actually much healthier-looking (or at least more tan) than it is when he's human.
 * Lydia Deetz on Beetle Juice is rather pale-skinned.

Answer: You actually checked? Wow, okay.