I think UV would appear as a new primary color. To describe it to the rest of us would be like trying to describe the difference between red and green to a person with red/green color blindness. It's all the same color to them. If it did appear as a new primary color, we would no longer have a color wheel, but a color tetrahedron with the primaries at each corner, the blending of two colors would form the edges and the blending of three colors would forn the faces.
Yeah. I think that would work.
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.