Methamphetamine in its pure form is colorless. However, most examples of the street drug are not. Here are some of the common impurities found in meth, and the colors they cause:

RED: The product was made from pseudoephedrine, and the red coloring of the tablet was not adequately washed away.
YELLOW/ORANGE: Ephedrine sulfate was used, and some of the sulfate was reduced to sulfur.
PURPLE: Iodine from a phosphorus-iodine reaction was not washed out.
GREEN: Copper (or other metallic) salts somehow made their way in to the mixture, probably due to the reaction vessel used in the manufacture.
BROWN: Oxidized red coloring, or tablating agent was present in the reduction.