What do primary colors look like? Serious question. I'm I guy, I "know" five colors. Not three...
More seriously, I see Red, Orange, Yellow, Green, Blue, Brown and Purple as distinct colors. I know intellectually that there are two sets of three primary colors (red green blue, and yellow, magenta, cyan) but in my personal 'perception space' I have the seven listed.
Well, there are only three primaries because we have three color photoreceptors. The other colors are a result of mixing signals of these three photoreceptors. A fourth receptor, for UV in this case, would yield a fourth primary and a colorspace with an added dimension. You also have bto keep in mind that color is a neurologicalperception in the brain and not a real, physical property. For example, two beams of light entering the eye may have completely different spectra in the visible region yet will be percieved to have identical color.