Note: This Q&A should have reviewed research done with molecular biology.
(a) Yokoyama S and Radlwimmer FB, The molecular genetics and evolution of red and green color vision in vertebrates. Genetics. 2001 Aug; 158(4): 1697–1710. http://www.ncbi.nlm.nih.gov/pmc/ ... 41/pdf/11545071.pdf
in the Introduction section: “HUMAN color vision is achieved through three types of photosensitive molecules: short wavelength- (or blue-) sensitive (SWS), middle wavelength- (or green-) sensitive (MWS), and long wavelength- (or
red-) sensitive (LWS) visual pigments, which absorb light maximally (max) at 420, 530, and 560 nm, respectively. However, having only SWS pigments and either MWS or LWS pigments, most mammals have dichromatic color vision. This condition
is commonly known as ‘red-green color blindness.’ Even in human, the ‘red-green color blindness’ is relatively common, affecting 8% of males.” (citations omitted)
(b) Jacobs GH, Evolution of Colour Vision in Mammals. Philos Trans R Soc Lond B Biol Sci, 364(1531): 2957–2967 (2009) www.ncbi.nlm.nih.gov/pmc/articles/PMC2781854/
Go to Table 1, where S, M, L are the same as in (a) above. For an animal to have trichromatic vision, it must have all three (S, M, L), such as howler monkey (“S, M+L”). Thus the text of this report said, "The howler monkey (Alouatta), on the other hand, is similar to the catarrhines in having two different X-chromosome opsin genes and separate populations of cones containing S, M and L pigments (figure 3a), allowing for trichromatic colour vision."
(c) Incidentally:
Caroline William, Many Animals Can Still See Colour in the Dead of Night. BBC, Dec 1, 2014 http://www.bbc.com/earth/story/2 ... see-colour-at-night
("Our [humans'] colour receptors [on cones] stop working when it gets darker than half-moonlight. At that point we switch to more sensitive, but colour-blind, receptors called rods. By adding together the output of all of these rods, we can still see, but only in shades of grey")