Iron oxide is just rust to you and me, but ferric oxide to chemists. Although it exists in abundance in nature, it is impossible to purify to acceptable safety standards. As a result, all iron oxide today is synthetically prepared. Depending on how iron oxide is prepared, it can develop different colors: yellow, red, brown and black. Different colors of iron oxide depend upon the oxidative state of the iron atom, with Fe+3 appearing brown and Fe+2 appearing red. Different colors of iron oxide also depend upon the amount of water added to the iron atom (iron hydroxides). This variation gives rise to the yellow, red, brown, and black colors.
Iron oxide is an insoluble mineral pigment. Consequently, its use in foods is limited. In comparison, iron oxide is used extensively in the cosmetic industry where it is combined with titanium dioxide to make a host of red, pink, beige, tan, and black colors. It is also combined with mica, making a wide variety of “pearlescent” colors used extensively in the cosmetic industry.