ANIMAL COLOR VISION
Humans are trichromats. Humans have three kinds of cones sensitive to long, medium and short wavelengths of light corresponding to red, green and blue respectively. The full gamut of colors is distinguished in the brain by comparing the strengths of three distinct signals. Dogs live in a monochromatic world. Horses are dichromats. Equine color space roughly corresponds to that of color blind humans. Turtles, reef fish and goldfish exhibit tetrachromacy. Pigeons have five different retinal pigments.
Tetrachromats and pentachromats are not simply better at seeing the same rainbow of colors that humans see. Chilean cognitive scientist Francesca Varela (1991: 182-183) describes the enigmatic quality of attempting to comprehend the color experience of other speciest:
It must be remembered, though, that a four-dimensional color space is fundamentally different from a three-dimensional one: strictly speaking the two color spaces are incommensurable for there is no way to map the kinds of distinctions available in four dimensions into the kinds of distinctions available in three dimensions without remainder.
Varela, Francesco. (1991) The Embodied Mind: Cognitive Science and Human Experience. MIT Press, Boston.
Roth and Kelber (2004) of Lund University in Sweden discovered “that nocturnal geckos can use cone-based color vision at very dim light levels when humans rely on color-blind rod vision.”
Diurnal lizards have four types of cone in their retina, sensitive to the visible spectrum plus ultraviolet. They do not have rods. Roth and Kelber studied Helmet Geckos which are nocturnal but have evolved from diurnal ancestors. Helmet Geckos do not have red-sensitive cones, the type least efficient in dim light conditions. Their three remaining cone types, sensitive to blue, green and ultraviolet are ten times larger than those found in diurnal lizards. Enormous size and enhanced light sensitivity are usually associated with rods rather than color discriminating cones.
Kelber and Roth conducted a series of behavioral experiments that confirmed that helmet geckos were using their modified cone anatomy to see in color at night. The geckos unfailingly distinguished between crickets fed with grey or blue tweezers of the same intensity in starlight and in dim moonlight.
Roth, Lina S. V. and Kelber, Almut. Nocturnal Colour Vision in Geckos. Proceedings of the Royal Society B: Biological Sciences, Volume 271, Biology Letters Supplement 6: 485-487. December 7, 2004
So far, the geckos and hawkmoths are the only two nocturnal species that have been shown in behavioural experiments to see in colour... Nocturnal colour vision may be far more common than anyone imagined and could be found in toads, frogs, bees, wasps, fireflies and creatures of the deepest oceans...
Palmer, Sally. Geckos: Under the Colour of Darkness. New Scientist, January 6, 2007, pages 36-39.