Rather than scumbling a pastel over the top of an existing block of color, try dusting. Hold a pastel over the block of color (it is easier if this has been fixed, but not essential) and scrape the stick's surface to create dust. Once you are happy with the arrangement of dust on the paper, use a flat palette knife to press the dust into the surface.
Tip 1: It is much easier to do this with the painting held horizontally - the pastel dust will fall exactly where you want it to, and not contaminate the rest of the picture.
Tip 2: Make sure the palette knife is really clean before trying this, and then even if you move the knife around a little bit it won't blend the colors together.
