I cheat and when i pipe as i am usually only doing letters or outlines, so it is usually in brown or white and i add some melted chocolate (either milk, dark or white) to my normal icing mix which i don't make overly runny. I find it works a treat.

Otherwise you can but the cans of icing (just like whipped cream cans, my son went to a party that had used them, the only thing was they were very sweet) that look like those professional cup cakes and piping icing in bags with the attachment already on it in various colours from the supermarket.

I hope that helps in some small way, have you googled it?