Im not an expert with tree frogs but im pretty sure the colour changing is a normal occurrence in the Red Eye. Follow a guy on instagram and if he takes pictures at night they are always a darker purpleish colour then they go back to the normal red and greens in his daytime picks. If you want to see some pics of some amazing tree frogs check out the_frog_keeper on instagram. This guy has a blue red eye tree frog, you should check it out. he also has some awesome feeding videos on You tube under the same name. Hope this helped. But I also think if he stays dark during the day that it could mean he is feeling stressed, again im not expert by far so hopefully someone else with more experience will correct me if im wrong.