I always carry both a blue and a red one. In my opinion anyway, blue generally works better on wood, felt, and aluminum, whereas red seems to work better on concrete and masonry. Plus I can always switch colors for those times when I say "oops, that doesn't look right"! Red also seems to be more messy, for some reason. I generally try to keep them in a baggie of some sort when I know that I won't be using them, just to help keep chalk of everything else, including myself.