My understanding, with Hyla chyrsoscelis at least, is that skin darkens as temps and light levels lower, and brighten with the reverse. I do know that new morphs tend to start out pretty lime colored. Unfortunately, all of that is observation of wild frogs.

I have noticed that when we build vivariums they tend to be much darker than natural/wild conditions.

The one set of captive Hyla versicolor/chrysoscelis I dealt with that stayed mostly green were set in a shelf opposite a northeast facing window and got indirect sunlight most of the day.