Some players seem to be a jinx, so if you have bad luck against them, its wise to make a note of it. They might even be cheating, so be wary of them. Keep a close watch on them, avoid taking risks against them, avoid getting involved with them, etc. The color codes are a simply way to do that (say, just flag them with red as a warning flag, if something bad happens when you are involved in a hand with them :-)