What colour is sad?

Blue is usually thought to have a sad quality. "Singing the blues."

Picasso's blue period seems sad.

Adding to what you said I've found that in the early 1500s is when blue started meaning sad,at that time it meant woeful.Also I have found that in the 1800s it meant depression.Although I have no idea why it means sad this might help on something.

I'm also adding~I think the color of sadness is violet; or blue-purple. You know why? Because blue is usually the stereo-type for sadness, and purple means drama. So purple with blue mixed together is dramatic sadness. Besides, pale violet looks like a very sad color to me. OR just plain violet.