The Flat Material Type is a very simple material type that produces an unshaded, perfectly even color over the entire part to which it is applied. 

This material is often used as a black-out material behind car grills or other meshes. It is also useful to create “Clown Pass” areas, which has uniquely colored flat materials applied to the different parts of a model so that these solid colors can be used to easily create selections in an image editing software. See Layers and Passes to automatically create a Clown Pass.