However, it can appear in different shades depending on the oxidation state and impurities present. For example:

* Wüstite: A common form of ferrous oxide, is typically black to dark gray.

* Iron(II) oxide (FeO) can also be greenish-black or brownish-black depending on the method of preparation.

It's important to note that ferrous oxide readily oxidizes in air to form ferric oxide (Fe2O3), which is rust. So, any ferrous oxide exposed to air will eventually turn reddish-brown.