Icing can be colored naturally with fruit and vegetable juices or powders. Juices must be added sparingly, as they alter the icing's flavor and consistency, thus imparting lighter colors than powders or concentrates.
If deep colors are the goal, it is often more effective to buy a commercial natural dye. Because the pigment concentration is greater, flavor and texture don't suffer. Natural store-bought dyes are used in the same way as their artificial counterparts. Red comes from beets, while yellow is a turmeric concentrate. Blue is very difficult to imitate, so it comes from a mixture of vegetable juice concentrates.