I am only new to the frog world myself, having aquired two juivenile (Australian) Giant Green Tree Frogs. From my little experiences and readings done on green tree frogs, they turn brown when they are stressed or unhappy. For example, if we get a really cold night here and the terranium temperature drops below 20 degrees celcius they will go brown, but colour up again once it warms up.
I havn't quite succeeded in keeping my fellas green all of the time yet, not entirely sure why, as they have everything that I understand that they need in all of the right conditions, so my next process is adding more hidding places. If that doesn't help, I will change their substrate as I noticed some rocks in their faeces yesterday morning (that would have hurt coming out )

Good luck getting your fella to colour up!