Hit flash
A hit flash turns the struck target a solid bright colour for a few frames on the exact frame of contact, so the hit reads instantly.
Also called damage flash, white flash.
The flash is the oldest trick in action games and still one of the most important: the target turns a plain, bright colour the moment it's hit. It has to land on the exact frame of contact. Even one frame late reads as lag.
How long should a hit flash last?
A few frames. A plain solid colour works better than anything fancier, and keeping it brief matters if your game also uses pale or white to mean something else, like a creature that can't be hurt right now.
The hit flash in Mytheria
A struck creature turns a solid cream for 8 frames on a normal hit, about 133 milliseconds, and 11 on the third hit of a combo. The flash lights itself, so at night it stays bright while the world around it goes dark. Hit marks use magenta and violet, colours no enemy warning uses, so a hit can never be mistaken for a danger sign.
The full post, with clips and demos How to make combat feel good