Yellow enough that I am concerned. No not as yellow as the smiley, but close. He has been acting weird today as well, chasing other fish around when he usually doesn't.
Erm i'm stuck then, i'd try PMing the-wolf tomorrow and see if he has any thoughts....
Unless its velvet? link...
Does the fish look kind of dull, has it been flashing/flicking at all?
What first came to my mind was breeding, z.danios chase each other when breeding but i didn't think there was a strong colour change involved (if at all).