A recent question on a forum that I belong to concerned what to use for icing/frosting colors so as to avoid artificial colors which are not good for you.
We use India Tree natural vegetable coloring. According to the website it's a concentrated vegetable liquid colorant. I do know that it only takes a tiny bit to color a bowl of frosting. The set is sold in a three pack of primary colors which can be combined to make any color you want.
If you want to switch away from petrochemical colorants this is a good alternative.