My 3" female Rusty crawfish has stayed this color blue now for 8 months and at least 3 molts. She's attempting to sneak-attack a male of the same species for the Pleco wafer he has. The male has kept his natural color over the same 8 months whereas the female changed. I find it unusual that despite having the same dietary intake an foods, one turned bright blue and stayed and the other didn't.
Notice my Northern Longear photo-bombed the pic. He get's very inquisitive when I approach the tank. He thinks I have food.