Thanks for the responses. The problem I'm experiencing is: We had a house built and the builder put durrock in the shower stall. The durrock was black in color. The tile contractor then put the tile on. Our problem is with the grout. The first go round, the grout looked in places like it was wet but did not change color when dried. Finally, the tile guy replaced the grout. The discoloration is not as bad but it is still there. Additionally, we're getting fairly substantial separation at the corners. Some could be settling but this seems to be excessive. I don't know what materials were used for tape, tile bond etc. The builder is not much help. That's why I'd appreciate any thoughts. Also, what happens if moisture gets behind the tile?