I don't remember ever dreaming in black and white... I do remember times where there has been colour in my dreams. Those times stand out. So I am assuming that any dream where colour did not stand out were actually in b&w.
The way I got to this conclusion is because of the way the cones in your eyes work. When there is minimal light, your cones no longer register colour, so at night, you're actually seeing in black and white. Your brain pretends it sees colour so you're not confused. It's very trippy when you realise. So in that same sense, I assume most of my dreams are void of colour, unless my dream-self thinks, "Oh look, there's a coloured XYZ". Otherwise it's just my brain-filter doing it's thing.