Humans tend to have pretty good colour vision compared to a lot of other mammals because we have three types of cones in our eyes, the things responsible for detecting colour, rather than the more common two. It's thought that this gave us an evolutionary edge as hunter gatherers as we were better able to spot fruit and berries. Red/green colour blindness is caused by a mutation that disables one of these cones, mostly in men. They still have the three cones, but one isn't working properly. Colourblindness is almost exclusively a male trait because the genes responsible for detecting red and green are both on the X chromosome. Men only have one copy of these (as they are XY), whereas women have two (XX), so if something goes wrong, women have a back up copy. But what if the mutation, rather than messing up red/green perception, shifted the range of colours it could detect? For a bloke, this wouldn't make a difference, he would see the same amount of colours, just slightly differently. However, if one of a woman's X chromosomes had this mutation, and one didn't, then she would theoretically be able to detect an additional rage of colours that most people can not. According to a study at Cambridge, around 12% of women have this mutation, but may not be able to completely demonstrate their super-vision practically. They did, however, manage to find one woman who could detect colours that others could not, making her a true tetrachromat. Incidentally, if you found that you were a tetrachromat by doing this test a while back, then, sorry to break it to you, but it's a load of drivel.
