Colour perception is a complicated business; most people have tristimulus vision - that is we perceive colour (broadly speaking) in reds, greens and blues. Your brain does the clever stuff of filling in the intermediate colours - if you're seeing a colour between red and green (yellow) the mix of stimulation of the red and green cones in your retina give you the sensation of yellow.
Although I do a lot of colourimetry (calibrating monitors, racking cameras etc) I was unaware that some people are tetramats (they have four sets of distinct cone cells) - they are more sensitive to the colours between red and green (and hence the opposite of men who have red/green colour blindness). Interestingly they are always women and invariably the mothers of men who are red/green colourblind. Dutch scientist HL de Vries discovered this in 1948 - here is an article in Discover magazine.