They're short colour formats you can use in Events, Scripts, and Welcome Messages.To see what color each number represents, join a game (or start one,) and use "/colortest".
I'm pretty sure Badspot based the default colorset off of those colors, because /c0 is red, /c1 is blue, /c2 is green, so on so on.
Yeah, I'm sure this was the inspiration to let us have red, blue, and green bricks.