It is possible, and quite common, in Magic for an object (card, permanent or spell) to be a single color, multiple colors or colorless. But it is not possible for an object to be both of a quality and not of that quality. For example, Doom Blade can target a non-black creature. A creature that is both green and black, like Glissa the Traitor, is still a black creature, so Doom Blade cannot target it. It does not matter that it is another color other than black, that does not make it nonblack. This principle also applies other similar requirements that reference card types; for example, artifact and non-artifact.