Tetrachromacy, or the ability to sense color in four dimensions, is common in some organisms (most birds are believed to enjoy tetrachromacy), but very rare in humans.  Our eyes use a cell in the retina called a cone to distinguish color, and usually people can see three primary colors; red, green and blue.  Consequently, we "normal" folk can easily distinguish 100 gradations of color, and about 1 million variations in hue.  With the extra cone, someone with tetrachromacy can see up to 100 million differences in hue!

Someone with this condition is called a tetrachomat.  Such people are thought to be extremely rare,  but so far, only women have been identified as tetrachomats, and in fact for genetic reasons, only women are thought to be able to inherit the ability.

To test yourself in your ability to distinguish hue, try to arrange the graph at the link here by gradation of color.

Not as easy as one might at first think!

Anyway, women, being a tetrachromat would give you a potential advantage as a doctor, or most certainly an interior decorator, like this woman, who is a suspected tetrachromat.

Genetic variation - it's awesome!