The color change drove me nuts when I first got a frog but I have two that do/did it. My tiger-legged tree frog did it for several months after I got it but now it just stays green. My White's does it all the time and it just seems to be something only they know. Best guess: when they are cool and stressed or hunting, they are brown; when they are warm and happy they are green. It all adds to the fun of watching frogs.