Very inaccurate summary, since I skimmed: Up to 12% of women have an X chromosome mutation that creates a fourth cone. This cone generally overlaps the spectral region covered by the cones sensitive to red and green. The exact area of overlap determines whether or not each woman sees &quot;more&quot;: If the fourth cone completely overlaps the region covered by an existing cone, no new information is presented to the visual system and the input is effectively discarded (she sees &quot;just as much&quot; red or green, not more, not less). If, however, the fourth cone has peak sensitivity in the gap between the cones covering red and green, she will she up to 100% more colour than women with normal vision.