'Skippers changing colour

I've noticed that my two mudskippers seem to be able to vary their body colour. At times they are so pale as to be almost white, with the blotches along the spine almost completely invisible. Does anyone know why this happens? Are they doing it on purpose for camouflage? Does it have to do with the air/water temperature? I'll try to post some pictures to illustrate what I'm talking about.

Frozen bloodworms, frozen brineshrimp and pinhead crickets... the color change happens really fast, so I wonder if it has to do with their mood/stress level.

I'm hoping that's the case. I spend quite a bit of time just sitting and watching them (I find them completely fascinating!) and when I noticed the color changing I was curious about what it meant.