EyeDrops looks perfect to use as a core engine to do the rendering as the user paints - you'd effectively be painting a shape file that could get fed into EyeDrops with each change and then show the result in the content area of the program.. nice.

My take is that if you were to make this hypothetical thing, the proper way to do would not be to make a new engine to add darkness/brightness capabilities on top of the shape file, but to try to extend EyeDrops (in a fully backwards-compatible way) and still use it.