P. regilla have 3 main color phases (there are likely others) that exist in any one given population: greens, browns, and intermediaries. The greens and browns usu. always keep their colors (may lighten somewhat, and appendages or laterals may take on coppery metallic hues at various light spectrums and temperatures. Intermediaries can change from green to brown, but rarely as dark a green or brown as the "non-changers". These 3 phases are a survival strategy that allows any one population to adapt to fluctuating environments (vegetation states, temp, etc). There might be less browns in a wet "green" year, or there might be less greens during a dry year...

In the case of the blue frog - P. regillas, which are green are so because white light bounces off of iridiophores (pigment cells), which is reflected as blue light; this light then goes through xanthophores (yellow pigment cells), which are at the surface of the skin, thus blue + yellow = green. Blue frogs have a recessive trait where the xanthophores don't work!