Do you dream in color? asks Mark-Jason Dominus. Interesting question, on which I've already some thoughts.
I'm colour-blind. I see some colours, but the words that are used to describe colours are for me largely arbitrary. Why use two different words, like, green and orange, for the same colour? A consequence of that is a difficulty to verbalize colours, which in turn makes it difficult for me to remember the colour of an object, if nobody told me what word to use to describe it. Without a proper vocabulary to classify them in my brain, I can't remember or percieve fully the colours.
So it shouldn't be a surprise that I dream "in black and white": or, more accurately, that I can't name and remember the colours of the objects that appear in my mind during dreams. Colours are an irrelevant part of my Weltanschauüng.
However, from time to time, I make a dream about a colour. Those are in general very simple dreams, focused on a single object; nothing happens; sometimes I only dream about a colour without an object. (Robert Louis Stevenson, in A Chapter on Dreams, says that he sometimes dreams about a particularly horrible and uncanny hue of brown.) And usually that colour is mauve, or the idea I have about what mauve should look like: a mix between red and blue, which does not exist for me in the real world.
I'm not sure how to explain this. Probably my brain is playing tricks to iself (that is, to me): my eyes are not able to send the signal mauve to the brain, but the brain circuitry is intact and is able to perceive mauve once the eyes are out of the loop. However, that new colour is so strange that it soon overrides all other aspects of the dream it appeared in. I don't have any other explanation (short of the Platonician thesis, that learning is remembering.)