I'm a new owner of two Whites Tree Frogs myself. When I bought my frogs in the pet store, they were actually green, when I took them home, during the car ride (store was over an hour away) they were brown and after a while they went right back to green again once they were back in their new home. I myself have not seen my frogs go back to brown since. They are some times "not as green" at night, when they go to soak in the water dish. Now from what I've read these frogs change color a lot. I've seen Whites in pet stores fairly often, they are USUALLY brown in pet stores, probably a lack of humidity?
To keep the temperature up I am using a 60w ceramic heat bulb located on the left side of the tank. I'm still not sure if this is the best way to keep the enclosure warm but it seems to be working. I keep my heat/humidity sensor in that general area and the warmest I've seen it is 80F, but I would imagine under the bulb where the frogs rest, it's closer to 85F. I know my frogs stay on the left side of the tank where it is warmer unless they want to explore/soak for a bit and they -always- sleep on that side of the tank under the bulb. I am sure there are other/better methods to heating the enclosure. For UVB (if the frogs do indeed need it) I allow my enclosure to get some indirect sunlight in the morning, since it's in the northeast part of my house near a window.
Now as far as humidity, I could be wrong here.. but I mist daily, the humidity never goes below 55%, my house humidity is 50% (which may be high). I don't think these frogs need high humidity. I think if you can keep humidity above 50% you should be fine. What I would do is add more foliage to your enclosure, things that will retain humidity better. In winter you could put a humidifier in the room to help keep your home from being too dry. I plan on doing this myself because I get chronic sore throats in winter, so it's win win!
Last thing about the pooping, you may want to hunt down a snake plant (Sansevieria trifasciata), they are quite common in store gardening sections. My frogs, love love love this plant. They always choose to sleep on the inside of the leaves and... they also use it as a toilet. I always find droppings on the lower part of this plant. In fact they are so fond of pooping on this plant I find the daily removal of feces from my enclosure quite simple, with paper towel.