Flash of Pain

In many video games, enemies that don't die from one hit will blink a different colour when damaged. Most of the time, it tends to be white, although it may come in different variants, like red.

The Flash of Pain will definitely be used on a boss that is Immune to Flinching, because it conveys that the player's attacks are actually landing damage. Especially important if the boss has a weak point on its body that must be hit in order to cause any damage (in which case usually just the weak part of the enemy will flash). Sometimes player can flash in a different color too when hit, when the game doesn't feature Mercy Invincibility.

Prolonged flashing may also signify that an enemy has Mercy Invincibility after you have attacked it, in which case you must wait for it to stop flashing before you can damage it again.

A related analogue occurs in first-person videogames, where because the player is unable to see their own character, if the player takes a hit the entire screen will flash momentarily to indicate damage.

This trope follows the Rule of Perception, of course, as the Video Game character presumably doesn't see his opponent flashing white for a split second.

Action Adventure

Bosses in The Legend of Zelda games.
- Link flashes in red, too.
Bosses and enemies in Shantae series either flash when damaged or not.
The enemies in the original Blaster Master games do this.
Lagoon applies this to the player character, Nasir, as well as enemies.

Action Game

Enemies with multiple hitpoints and bosses in Tank Force flash white.
Bosses blink white Last Alert.
Larger enemies, bosses and walls in Heavy Barrel

Beat'Em Up

Many Konami games do this when bosses are hit combined with turning red such as Teenage Mutant Ninja Turtles the Arcade Game, The Simpsons, and X-Men.

Fighting Game

This shows up occasionally in the Capcom vs. Whatever series.
- In Marvel vs. Capcom, certain characters, like the Hulk, could take a shot without flinching, flashing red to show they did this. Hidden character Mecha-Zangief could take an infinite number of shots at the cost of being unable to block.
- In Tatsunoko vs. Capcom, the giants and Yami do. This is because giant characters, who fight alone, don't flinch when damaged (as compensation for not having a partner).

First-Person Shooter

In the Marathon trilogy, S'pht compilers not only flash into static when they're hit, that's also their death animation.

Multiple genres

The enemies in platforming parts of ActRaiser blink red and white when hit. Even those which require single hit, blink red before exploding. In town building parts, the enemies blink white when hit.
Vice: Project Doom: The enemies blink yellow when hit.
Enemies and bosses in Spin Master blink red when hit.

Platform Game

The Mega Man series of games; usually they will flash white.
Many enemies in Metroid. Sometimes, a boss will flash yellow when an attack will just stun it, and red when attacks start to damage it for real.
Most bosses in Kirby games, along with some stronger enemies.
All of the Contra games have the enemies flash when your shots are dealing damage, but in Contra: Hard Corps the bosses and minibosses go through an almost psychedelic pattern of colors while taking damage. It's especially unsettling with the larger bosses.
If a boss in Moon Crystal is hit, the entire screen flashes white.
Bosses and some other things with multiple hitpoints seem to blink white in 2D Sonic the Hedgehog games.
Gish flashes red in his game when he is damaged.
The enemies in Batman: Return of the Joker blink red and crates blink white after getting hit.
The enemies in Journey to Silius flash pink after getting hit for some reason.
Bosses in the first Adventure Island game blink purple when hit. Subsequent games use white color instead.
Strangely enough, enemies and bosses in Bug!! only flash red in the Sega Saturn version. The PC version subverts this.
Enemies in Dynamite Headdy always flash white with the same little sound effect whenever they take damage without dying.
Enemies in Ratchet and Clank flash red when damaged.

Role-Playing Game

Most enemies in Chrono Trigger.

Shoot'Em Up

Enemies in Radiant Silvergun flash white when hit.
Character in Mushihime-sama series flash yellow though.
Enemies in Don Pachi series have been known to flash different color depending on game. In one game, they flash blue while in the other, they flash gold color.
In Spheres Of Chaos, it's possible to add an option for the whole screen to flash when a black hole is hit.
Enemies and bosses in Zanac Neo.
Flashes of pain often happen in Zero Wing.
Bosses in Super Space Invaders / Majestic Twelve flash when hit on a vulnerable area.
Bosses and some enemies in arcade Bells And Whistles flash when hit.

Wide Open Sandbox

Most mobs in Minecraft jump back a bit and flash white when you hit them.