What color lightsaber would you want?

Grey light? I kind of find that to be a stretch. Grey, as a color, is a blend of both white and black. Just from looking at the visible light spectrum, I've never seen something grey in there. It would have to be at the extreme low end of the light spectrum. And humans can't even see ultra-violte light. I would expect grey would be even lower on the spectrum than that. Wouldn't a lightsaber whose blade you couldn't see be even more dangerous?