I am not quite sure how much an under paint gives me - this one was done with purple values and rubbing alcohol. The picture on the right has some blues and yellows layered on top of it.
So I am not convinced about the advantages of under painting and when or not to use it.... You sure use less pastel because you don't have to rub in so much chalk to cover all of the white spots in the sand paper.
The key to realistic clouds is to paint the sky into the cloud - negative painting. It gives the clouds their typical transparency and "cuttony" appearance.
I got some compliments on this one. It started as a study and it turned out to be very soothing to the eye. I love the fact that there are so many blue transitions. Now I am going to have to frame it!