One way might be to use a patch with vertex interpolation, as in this example.
EDIT:
Here's how you might apply that to a circle:
t=linspace(0,360,500); t(end)=[];
x=cosd(t);
y=sind(t);
c=cosd(t);
patch(x,y,c); colormap(gray); axis equal; caxis([-0.7970 ,0.6373])
