In some plants, a red pigment, cyanidin, is synthesized from a colorless precursor. The addition of a hydroxyl group (-OH) to the canidin molecule causes it to become purple. In a cross between two randomly selected purple plants, the following results are obtained: 94 purple: 31 red: 43 colorless.

How many genes are involved in determining these flower colors? Which genotypic combinations produce which phenotypes?