Iodine compounds can be used in X-rays because X-rays cannot go through them easily. Some metals are purified using their iodides. Silver iodide is used in photographic film and cloud seeding. Erythrosine is a food coloring that has iodine in it. Iodine can be used to make illegal drugs so it is restricted in some areas. Iodine can be used as a test for reducing agents. Iodine turns colorless when mixed with a reducing agent. Iodine also turns black when mixed with starch. Iodides become colored when mixed with oxidizing agents.