Many color words, such as blue, can be used as a noun, a verb and an adjective, although the verb sense of most of them merely means either to make something that particular color or to describe something taking on that color. e.g. The newspaper yellowed with age, or The metal on the rifle was blued. The adjective forms of color words take on more interesting meanings in addition to simply describing an object of a particular color. He was in a blue mood. He was green with envy.