I've got 5 yoyos and 8 striata in my 75. The yoyos' color intensity changes whenever they change their state - playing, active, inactive, chasing, chased, messing with the gouramis, messing with each other, exploring, sleeping. I look at it like blinking. (Incidentally, My opaline gourami does more or less the same thing. He's pale blue when he's sleeping or fighting, and he gets his scribbles back when he's awake or happy.)
I've noticed that my Botia kubotai also pale out when they do the loachy dance. They go from mahogany to tan while they fight, and then go right back to normal once one of them has had enough.