Dichromatic vision requires two spectral receptor types, where Trichromatic vision requires three. It is thought, trichromatic vision is the result of a gene duplication of the dichromatic spectral receptor types ( Surridge A, K., et al 2003).  

What is Gene Duplication? 

Gene duplication is an important process in new genes and genetic diversity among organisms. Genetic diversity is differences in genetic makeup within a species and also among traits of organisms within a species.  Genetic diversity within a species is strongly associated with that species ability to adapt, reproduce and survive in their given ecological niche. Gene duplication can result from unequal crossing over, retroposition or chromosomal (or genome) duplication (Magadum, S et, al., 2013).  In the case of the dichoromatic to trichoromatic vision, a photorecptor was most likely mutated, and thus the new photorecptor sensitivity adapted to the light differntly than prior. This trait became the key to trichromatic vision and was passed to offspring. 

Connecting dichromatic and trichromatic vision to color blindness is another way evolutionary developments have altered genetic material in the eye and is seen today across a variety of humans.  The M and L cones are seen to hybridize, which is a reaction that occurs when there are various copies of the M gene, but only 1 gets transcribed ( Surridge A, K., et al 2003).  The result of the hybrid gene is, anomalous trichromacy!  Anomalous trichromacy is what we modern day call a type of color blindess. Because this is affecting the L and M cones, this is red / green colorblindess. If you recall from the trichromatic page, L and M cone signals interpret red-green photopigments  ( Surridge A, K., et al 2003).