How do you explain a colour to a blind person?

Depends on the person or what you're describing. Some colour blind people can see certain colours, and they usually learn to recognize what colour something is on their own.  

Basically do something such as using touch to describe colours. Have the person hold certain objects while you tell them what colour it is. It might be helpful to consider using objects that are almost always a certain colour. 
Consider smells and tastes to describe colours. Smells and tastes can definitely be associated with certain colours.

Very cool! free points wooo