It's kind of cool that the visible light spectrum is only a small fraction of the entire spectrum of light.  It makes up the range of colors that we can see basically because we have receptors to be able to see them.  Imagine if we could see the entire spectrum.  There would be like a ton of other colors.  That would be really cool.  

Sometimes I try to imagine a color I've never seen, but it's pretty much an impossible task.  I only know colors in terms of what I can see, so it's frankly outside my capacity to picture something else.  But it's fun to try sometimes anyway.  It's kind of like imagining extra spatial dimensions.  I can't think in those terms because I don't exist in them and have never experienced them, but it's still kind of fun to try.

On a side note, I feel bad for people with color blindness.  There's a whole vibrant part of the world that you're just missing out on.  I wouldn't like that.