One of our members Geoff asked a great question regarding apply white over dark:
When at times I need to go back in with White pastel 101 over a dark colour I find the Faber Castell does not cover strongly enough. Is there an alternative you would recommend?
Unlike soft pastels the pastel pencils do not work well when light colours are placed on top of dark tones, this is the reason I always state that they should be worked light to dark. The make up of the pastel pencils means that they are finer than soft pastels therefore do not have the strength of pigment that would allow the dark to light rule to apply.
Once you understand this principle and get used to working from light to dark this is no longer presents a problem. I have produced hundreds of pastel pencil paintings over the 30 years I have been using the medium on an amazing variety of subjects and am now convinced that the pastel pencils are truly a master medium.
To answer the question, there is no satisfactory answer that would allow white to be placed over a dark tone. If need be the dark colour should be erased completely then the white re-applied.