Well, keep in mind that Knight anoles and green tree frogs aren't supposed to live in the same places, even if they sometimes do. So, if you bought a green tree frog, it's probably from somewhere North of where Knights have been introduced, anyway, so their environments are different.
Anyway, a brown frog is usually a cold or stressed frog. Sometimes, when they're sick, they stay brown for a long time. If it's only brown a little bit of each day I wouldn't worry, but if it spends most of the day or night that color there is something likely wrong.
It could be the Knight anole (who will eat that frog!) or something else, but I'd quickly separate them. If it's only one GTF, just buy it a separate 10-gallon tank, they're pretty cheap. The lids are the most expensive part, but you can even make one of your own using screen and clips
Remember, most GTFs and Knights are wild caught and will transfer parasites to one another. And they're certainly not happy being near eachother, so I'd separate them 
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Got hips like Cinderella / Must be having a good shame / Talking sweet about nothing / Cookie I think you're Tame 