The light spectrum doesn't appear to go from white to black to me. I might be wrong, since I haven't learned about the light spectrum since elementary school, but white is the color we perceive when we're given all of the other colors simultaneously. Black is the color we perceive when given none of the colors. They colors tend to get darker from the right to left.

On spectrums, ultraviolet is usually drawn as a violet color, but it isn't really that color. The fact that it's "beyond the visible light spectrum" means we humans are incapable of perceiving it.

Even different animals perceive different colors differently, so colors don't have objective ways of being perceived. The experience of color isn't in the light itself, it's what our brains imprint in our minds when our eyes are exposed to certain types of light. So there isn't even really an objective color that exists, and that we would perceive, if we could. To our brains, there simply is no such color.

But you could say, if our brains had evolved the ability to see ultraviolet, what would it look like? Looking at the spectrum pattern near the left, it would make sense that it would appear to be something like a dark purple, but not that exactly, since you're incapable of imagining whatever color it is.

It would be kind of nice to have synaesthesia, to see what it's like. I think everyone associates dark colors with low tones, and light colors with light tones. I do associate certain colors with certain letters. A is red, B is blue, C is yellow, D is red... but I don't actually hallucinate imprints of the colors when I see the letters, as synaesthetic people do. Except for in the strange case of the word Google. When I see it, in the font that Google is usually typed in, I'm so used to seeing it typed in colors, I might be experiencing what synaesthetic people do.