Dolphins are colorblind, so they do not see colors. In theory it is possible that they have a limited form of color vision in some light condition.
Let me explain you why they can’t see colors.
To be able to see colors, the retina should have at least two different kinds of cones with different sensitivities. Humans have 3 types of cones. Most mammals have two different types of cones, but not dolphins. Dolphins have only one type of come: L-cones, this is why they are colorblind.