Flash currently attacking enemy

[Entroper]

Should be a "simple" palette swap on the nametable.

[latzio]

Individual enemies overlap NT boundaries so this isn't trivial. Indeed sometimes in the vanilla game one enemy's palette can be used to draw parts of another even. I think this happens without much obvious effect in the Sea Shrine.

[Kronoan]

This might be a limit of the technology of the NES itself sadly. They could implement this in remakes probably because of the better ability of the better hardware to do certain effects. Although, if anyone can figure out a way to do this it's you Entroper! :)