[cjak]

I use an analogy with perfect pitch to illustrate colour-blindness to those with regular colour vision.

People with perfect pitch can be played a note on a piano, and identify it sight-unseen. "That's a C-sharp", they'll say confidently. Most others can tell if a note is higher or lower than another, or can pick an interval.

Regular colour vision is perfect pitch for colours. Most folks can identify a blue or a green in isolation, but some like me can only see relationships between colours (one is "bluer" or "greener" than another).