Most people (or so it was believed) have three different types of cone cells in the eye to detect color.  Each type responds to a different set of wavelengths of light, and all the colors that we see are interpreted from the activations of the three different types of cone cells.  Now it turns out that many women (possibly more than 50%) have a fourth type of cone cell.  These women are able to detect a wider array of colors than other women or men with the standard 3 types of cone cells.  Pretty crazy.