Even during winter months when ambient humidity is terrible in most of the US, you can always provide a humid hide when misting isn't enough. Get a Glad/Tupperware (with lid) big enough for him to fit in and stuff it with dampened (doesn't drip when squeezed) sphagnum moss. Cut a hole in the lid or side so he can get in, but the moisture can't escape quickly. Make the tank changes recommended by the others, but get him a humid hide ASAP so he can begin rehydrating. His color change could actually indicate that he has an entire shed stuck to him, and he's hissing because he can no longer see what the huge warm thing is that is passing by him, and translating it to be something dangerous.