Having had a green stylo that bleached, I've asked several questions here and also read up on it.
An sps that is underlit but otherwise has a good environment will lose color brightness ans start turning brown. It will lose the effectiveness of it's zooxanthelae, and start getting nutrients from the water instead. Obviously, if it is severly underlit, it won't survive at all, and just turn brown and die.
The green stylophora that I got was under PC's at the lfs, and had a distinctly brown skeleton and green polyps. I put it under my MH, and the brown went away. Unfortunately, so did some of the polyps, and the skeleton actually turned white. I had light shocked and bleached it. I now have it under a screen, and the polyps are slowly coming back greener than before, and the skeleton is tan colored rather than brown.