Um red maybe?
I don't think they are able to image colors at all since they are blind and do not realize what they look like. I'm not really sure though since I'm not blind.
Most blind people actually know what colors WOULD look like. But their opinions differ between person. Some, may not have any clue at all but some do. When you dream, (People who see) you actually dream in hazes and colors that you may have seen before you go to bed. I have a blind cousin who says she dreams in like a light reddish color and sometimes people who havnt been blind all their lives (some may even when blind when they turned like 1) can dream in all colors. It all depends on the brain. At least thats what I have read about.