There's a subset of women who have four cones (instead of three) in their eyes, enabling them to distinguish between colors that nobody else can. Most of these women probably have no idea that their vision is unique. For context: The human retina (usually) has three cone receptors that, roughly, are tuned to sense short, medium, or long wavelengths. The brain "sees" an image by comparing how activated these three sensors are. It is the ratio of activation that determines the color we see. Hence, if you were to look at a pure orange light, and then look at a light made by mixing red and green lights, the two would look identical to your brain even though they are made in different ways. This partly explains, for example, why our phone screens only need three kinds of pixels (red, green, blue) to convince the brain that it is seeing every possible color. Now, the genes for the medium- and long-wavelength cones sit right next to each other on the X chromosome. Sometimes, when eggs or sperm are made, they can "swap over" each other and make a hybrid gene that is tuned to detect light *between* those two wavelengths. A man who inherits a hybrid gene on his X chromosome is called an "anomalous trichromat," and will generally have trouble telling apart reds and greens. But his mother, carrying an ordinary gene on one X chromosome and a hybrid gene on the other X chromosome, is different! Early in development, cells in the retina randomly silence one of the X chromosomes, and its descendant cells then inherit that selection. Since there are millions of these cells in the retina, each of those cells will express the gene on either this X chromosome or that X chromosome. This means that, in these rare women, their retina will actually contain *four* types of cones rather than three. In 2010, researchers tested the vision of women who have sons with red-green colorblindness, and soon found a woman -- who they called cDa29 -- that seemed to possess four cone cells. They tested her child with an anomaloscope, an instrument that projects a small circle split across the middle. One half is lit by a single pure orange wavelength, and the other is lit by a mixture of red and green light. A knob is used to control the ratio between the colors, and the participant is asked to turn this knob until the two halves look exactly identical. The son was red-green colorblind. Then they showed the mother color pairs that look identical to most of us, but look different to a person with a fourth cone. She could quickly spot color differences that nobody else could. cDa29 was later sequenced, and researchers found that she had normal cones for long-, medium- and short wavelengths, but also a hybrid gene made by merging the first four exons from the middle-wave cone, and the last two exons from the long-wave cone. She can see (and distinguish colors) between these two wavelengths. Article snippet from a recent New Yorker article where I first heard about this.