When two or more things are contrasting, they are different from each other in a way you notice — often a way that looks interesting or attractive. A black frame on a white wall; a quiet person next to a loud one; sweet and salty in the same dish. Teachers, writers, and people who care about clothes or design use this word a lot. It comes from the verb contrast. You can also meet contrasting as the ordinary -ing form of that verb (they are contrasting the two photos), but as an adjective it means set apart by a clear difference. Neutral, not slang, not especially formal. Though actually the difference should be one you can feel or see; it is more than just “not the same on paper.”