You would have to record the handle of the image object returned by imagesc(), get() the CData property from that handle, set the diagonal of that color data to be equal valued (e.g., all 1) on all three color planes, and set() the CData back to the modified matrix.
This will not work for pseudocolor images, though; right at the moment I do not have access to MATLAB to test whether imagesc() creates truecolor or pseudocolor CData . If it does create pseudocolor, then it becomes more complex, especially if white is not one of the colors in the color map.
