This article is from the Dolphin
FAQ, by Jaap van der Toorn
The short answer is: yes, they can. Just like people can visualize an object by just touching it, dolphins can get an idea of what an object looks like by scanning it with their sonar. They can also identify objects with their sonar that they have only been able to see. If they form a visual picture from the sonar information (visualization) or form an acoustical picture from visual information is still unresolved. This capability is called cross-modal transfer and it has been demonstrated in only a few animal species so far: the bottlenose dolphin and the California sea lion. See the following references for more details on this subject.
R.J. Schusterman, D. Kastak & C. Reichmuth (1995) Equivalence class formation and cross-modal transfer: testing marine mammals. In: R.A. Kastelein, J.A. Thomas & P.E. Nachtigall (eds): Sensory systems of Aquatic Mammals, pp. 579-584 De Spil Publishers, Woerden, the Netherlands ISBN 90-72743-05-9
A.A. Pack & L.M. Herman (1995) Sensory integration in the bottlenosed dolphin: Immediate recognition of complex shapes across the senses of echolocation and vision J. Acoustical Society of America 98(2) Part 1: 722-733
2.7 - 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