"Sharks and rays do contain both rod and cone photoreceptors; however rays possess two cone opsin genes whereas sharks have only one cone. Sharks therefore were

"Sharks and rays do contain both rod and cone photoreceptors; however rays possess two cone opsin genes whereas sharks have only one cone. Sharks therefore were found to have lost the ability to see colors." Surely if you have both rods and one type of cone, you do still have some degree of colour vision, from the difference between the rod and cone response curves? Either the rods and cones both respond (one end of a colour spectrum), or just the rods respond (other end of the spectrum), and colours in between can be distinguished to some degree by the relative differences as the cone response progressively drops off.