A study published in the most recent issue of the Journal of Experimental Biology has shown that coral reef fish are capable of colour vision.
Ulrike Siebeck, Guy Wallis and Lenore Litherland trained groups of freshly-caught (held for only 3"4 days after capture) damselfish Pomacentrus amboinensis to tap a rewarded colour stimulus with their mouths at least ten times before they received a reward in the form of food introduced via a feeding tube.
The researchers carried out two different sets of experiments using blue and yellow stimuli of different shades, and found that the fish were able to discriminate between blue and yellow (i.e. the blue-trained fish ignored a yellow stimulus, and vice versa), and were able to do so independent of the brightness of the stimulus (a conclusive test of colour vision in animals).
The authors surmise that in order to distinguish blue from yellow, P. amboinensis must have at least two photoreceptors with different spectral sensitivities, and are thus capable of colour vision.
For more information, see the paper: Siebeck, UE, GM Wallis and L Litherland (2008) Colour vision in coral reef fish. Journal of Experimental Biology 211, pp. 354"360.