fill() creates a patch so it is limited to what a patch can do. For that see http://www.mathworks.com/help/matlab/visualize/specifying-patch-coloring.html
Note that to do the interpolated coloring for faces you need multiple faces, whereas fill() only creates one face. You will be wanting to create a triangular mesh or quad mesh to color more closely.
Or you could create an image of the colors you want and apply the image as a texture map over a surface object created with surf(). You might want to use this File Exchange contribution
