shadedpcolor displays one dataset in color, and a second by varying the intensity of the pixels in the image. We use this for displaying the vertical gradients in a field on top of the field itself. It really makes the field more three-d. A similar effect can be achieved by using lighting, but this is much more deterministic.

For the above picture I ran:

shadedpcolor(x,z,U,(dU),[-1 1]/4,[-1 1]/300,0.7,cmp,0);
axis ij;
shadedcolorbar('v',[-1 1]/4,0.7,cmp);