What kind of lighting is he getting? My frogs have 6500K T8 grow lights for plants on their table and the normal incandescent room lighting, plus a blue 60 watt daytime bulb over their housing and red 60 watt bulb at night; all lighting is 12 hrs on 12 off. Jelly Bean and Honey-Lime change color the most infrequently. Honey is almost always lime colored (honey color is his version of brown), and Jelly is a pretty Hersey's chocolate brown I'd say 98% of the time, and an olive green on occasion (mostly goes green at night, oddly). Shirley is my color change expert. In the daytime, whether awake or asleep, she is varies between greener seafoam green to pale blue-green, at night she turns that purplish-brown color you mentioned... when transitioning it is common that the sides and legs stay green while the top side is the darker color. She's never turned as brown as a brown as the pics of your guy. She will on occasion turn a smokier purple grey color, which to me looks like a zombie skin-- bleh! She is mostly active when she is darker since she is hunting, but I have seen her inactive and soaking for extended periods of time where she was the zombie color. My guys all have white paper toweling for substrate, Shirl & honey have a purple soaking dish and upside down saucer, a pale green flower pot and darker green silk leaves for their plant. Jelly has a marbled green and while soaker dish, a pale green aucer and flower pot, and a single giant medium green silk leaf for his plant.

Your humidity sounds fine to me, though 50% is better than 40% of course. Temps should be fine, mine average 77-80 in the daytime and 73-75 at night. I'd try to experiment with the lighting.