It's a fourth cone in the eyes. Interesting that the brain just rolls with it being present. Something similar was done to give squirrel monkeys the missing cone they need to see red: https://www.nationalgeographic.com/science/article/gene-therapy-gives-full-colour-vision-to-colour-blind-monkeys

Eyes.

Several of the most common types of colorblindness are much more common in men because they're X chromosome linked. Women have to fail the lottery twice in a row to be red/green colorblind, men just have to fail it once.

Men who are red/green colorblind have mutated green cone cells, which respond to lower wavelengths than they should, closer to yellow than green. So red and green look more similar than they should.

Women who lose the genetic lottery once but not twice may have normal green and abnormal "yellow" cones in their eyes. In many cases, the "yellow" cones just don't do anything, and they have normal trichromat vision. A very rare set of women have both green and active yellow cones...that detect a wavelength that isn't useful to detect because red and green cones working together can do it just fine, so they don't really see anything else. 12% of women aren't walking around seeing turbocolor.

The eyes. Tetrachromats have a 4th kind of cone in the eye.