The cells in the back of the eye that sense light, called rods and cones, normally only detect 3 different colours; red, blue, and green. Any other colour that we see is a mixture of these three colours that is processed by the brain.
Because human vision is based on three colours, we are considered to be trichromats, with tri meaning three, and chrom meaning colour. Some other animals, like monkeys, mice, and dogs, only sense two colours, called dichromats, and perceive far fewer colours overall as a result. Even some humans have a condition, colour blindness, which affects vision in the same way.
Just as some people have a rare genetic disorder that causes them to sense fewer colours, some people also have the opposite issue. Tetrachromats are capable of seeing colours that most humans can’t even imagine. And not just a few colours, either. Where the average person can see approximately one million different colours, a person with tetrachromacy can see approximately one hundred million colours.
This type of vision has been known about for a long time, as it is natural for some animals, such as mantis shrimp. But it’s so rare in humans that it can be difficult for scientists and researchers to find anyone with the condition. Currently, one such person lives in Northern England.
For privacy purposes, her name is not publicly available, but she does have a rather cryptic research code-name of cDa29. Her cooperation with researchers helps them to better understand what the world looks like through her eyes, but that process can be rather difficult. Her trying to describe the colours she sees is very much like trying to describe any one of the colours seen on the basic rainbow to a person that’s been blind since birth.