I have found the lighter color their surrounding the lighter color they will be. I have two of them and have changed my tank around many times. I now use white rocks under the water with light colored stones for them to rest on. I put down green sheet moss on top of some brown and grey platforms that I use for the land section. If they stay in those areas they are bright green. When they climb upon the dark brown back wall and rest there in the waterfall they will turn completely dark green. One of them will retain a couple of lighter green splotches but mostly dark. They spend most of their time at the bottom in the lighter color environment so they are usually bright green. I do not use any additional heating just room temp. My belief is that they lighten or darken just to camouflage themselves.