How would you describe a color to a completely blind person?

I’m constantly trying to think of how to describe things to blind people.
I met a little boy at a disabilities conference I was doing childcare for who felt my hair and told me that it felt like purple.

But how do you describe a color to them? Or tell them what color something is?
I feel dumb, but I’m trying to understand more about disabilities, as I want to be a special ed. teacher. Blindness is the one handicap that I don’t have one-on-one experience working with.