You mean to say feces, like waste, and not faces, right? General health can often be determined this way I think. I know clear can often mean an internal parasite. This is from FW knowledge, but I think it would transfer to SW.
A good guideline would be whatever color it goes in, should be similar coming out again. If you feed foods that do not have much color like scallops and such, that will not be the best indicator. You should also look for whether the wastes are solid or liquid. The liquid or "loose" wastes would also indicate a possible problem.