Can dolphins see colors?

To able to see colors, the retina must have at least 2 different kinds of cones, with different sensitivities. Most mammals have 2 types of cones: L-cones (sensitive to long-wavelength light, red to green) and S-cones (sentitive to short-wavelength light, blue to violet or near UV). Humans and some other primates have 3 types of cones, giving them a better color vision. Only a few landmammals have only one type of cone, which means they are colorblind. All these landmammals are essentially nocturnal animals.

Whales and dolphins (as well as seals and sea lions) have only one type of cone: the L-cones. Although these cones are more sensitive for short-wavelength light than the L-cones of terrestrial mammals, they still have a very low sensitivity for blue light. And because there is only one type of cone, they are essentially colorblind (although in theory it is possible that there is a very limited form of colorvision in some light conditions, when both the rods and the cones are active).

L. Peichl, G. Behrmann & R.H.H. Kröger (2001)
For whales and seals the ocean is not blue: a visual pigment loss in marine mammals
European Journal of Neuroscience, vol. 13: 1520-1528