revil

If you have 2 images, one a non-uniform shade of the other, is it possible to derive the exact transparent layer so that image1 + transparent layer = image2 (not through trial and error, but through a formulated process)? More technically, if you have 2 colors and their rgb/hsl values, is there some known formula that yields the rgb/hsl and alpha values of the 'difference' of the 2 colors? Thanks!