Um red maybe? I don't think they are able to image 색깔 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 색깔 WOULD look like. But their opinions differ between person. Some, may not have any clue at all but some do. When 당신 dream, (People who see) 당신 actually dream in hazes and 색깔 that 당신 may have seen before 당신 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.