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.