One study suggested that 2–3% of the world's women might have the kind of fourth cone that lies between the standard red and green cones, giving, theoretically, a significant increase in color differentiation. Another study suggests that as many as 50% of women and 8% of men may have four photopigments.
This isn't the way that color vision works. It's easy to be reductionist and think that each type of cone in the retina gives us one type of phenomenal color experience. However, subjective color phenomena are not generated at the input level of the retina, but later, in the visual cortex. Once you recognize this, there's really no reason to think that tetrachromats can see colors we don't have names for, or that dichromats are unable to see red and green.
We have cells that detect three colors; that's why we perceive three primary colors of light. Combinations of excited cells produce the shades that lie between. But the relatively low number of primary colors is what makes them special; if we could see more, it is likely that we wouldn't assign as much cultural importance to each one.
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