Mine change all the time, I have seen no factors that cause this, but they do darken before shedding and when hanging out in darker terrain. They may have normally done this in nature to elude predators, but pets seem to just change at random... like anoles.
I guess, if they feel secure, they may stay bright on dark terrain, but I think they might not have full control over it. (Don't worry, color changes are not an immediate sign of any one specific thing. EG, mood, camo, or sun bathing, or sickness.)