http://members.tripod.com/yumaparanormal/night_hag.htm there's a good explanation of sleep paralysis or "night hag" dreams;the ones I've had are so vivid, and scary! I can remember most of them;although I usually dream in color, with taste,smell, touch activated, etc.
One hypothesis is that if you often have s.p. you might want to get checked out for sleep apnea. Do you snore a lot? Wake up not breathing? Just an idea. I haven't had any night hag dreams since I started on my machine for sleep apnea.