What is it like to be a tetrachromat?

Most people are trichromats, but some people have an extra kind of cone cell in their retinas: they are tetrachromats. Recent science shows that some tetrachromats make distinctions between reflectances that ordinary viewers are unable to detect. This paper asks what kind of colour experience these tetrachromats have. It rejects the claim that they simply see finer gradations of those colours familiar to trichromats. Instead, it draws on both scientific evidence and anecdotal data to argue that tetrachromats’ visual acuity is due to seeing novel colours: elementary colours that trichromat viewers are unable to see or imagine.