Ferrous oxide, also known as iron(II) oxide, is typically black in color. However, the exact shade can vary depending on the method of preparation and the purity of the compound.

Here's why:

* Iron oxides are polymorphs: This means they can exist in different crystal structures, each with slightly different properties. Ferrous oxide can exist in a range of forms, including wüstite (the most common), which is black, and other forms that can be greenish-black or even reddish-brown.

* Impurities: The presence of impurities in the sample can also affect the color.

* Oxidation state: Ferrous oxide can readily oxidize to ferric oxide (Fe₂O₃), which is rust-colored. Therefore, if the sample is not properly stored or handled, it may appear reddish-brown due to the presence of ferric oxide.

Overall, while ferrous oxide is generally considered black, variations in its color are possible due to factors like crystal structure, impurities, and oxidation.