Spock Color

Spock Color
If a person had no senses or emotion but could understand you logically how would you describe color?

For instance, suppose you had a visit from Star Trek's Spock kind of family from another planet and they did not have emotion, sight, hearing, touch or smell but had had these things understandably described to them already except for color how would you describe color?
Any answer is a good answer so please give it a try.

I personally don't think it would be possible because there would be nothing to reference to.