You can describe purple by putting their hand or something on cold metal. =]i did this to my grandpa who turned blind at the age of 13. being in darkness so long, he forgot colors so he asked me to...
I think it is nearly impossible to describe any color to a blind person, unless they weren't born blind. They might understand the color black because that is all they see.
Well I dont know if you actually know someone who is blind, but, i found this website and im sure it could help answer your question.
http://www.wikihow.com/Describe-a-Color-to-a-Blind-Person