Dr Marshall has examined the eyes of 100 bird, fish and other sea creatures, looking for differences in their physical structures. He then studied the behaviour of these animals to see how they use their vision. To a human, yellow is one of the most important colours because we are primates and have evolved to be aware of the colour of ripening fruits, such as bananas. But reef fish that are all yellow do not look like that to those that prey on them. Also, because fish have poor visual resolution, prey that have yellow and blue patterning merge into an inconspicuous grey.
Humans have three cones in the eyes for detecting colour, whereas most reef fish have only two.
An exotic goldfish, on the other hand, has four cones, including one that can see colour in the ultraviolet range. Australian parrots also have four colour receptor cones in their eyes and two ultraviolet hues.