If bạn had to describe a color to someone who's been blind since birth, what would bạn say?

I have no idea what I would say.
"It's red."
"What's 'red?'"
"You know....red...like blood..."
"What does blood look like?"
"FUUUUUUUUU-"

(I'm not making fun of any blind people, I'm just curious. :P)