I'm playing with a related problem in my spare time - braille character-based color graphics; while we have enough precision for sharp edges, the fundamental issues with color are the still the same: if we begin with a supersampling pass for assignment, we lack precision, so we may need to do some contrast fixups afterward. I think some contrast enhancement based on your sampling schemes might be useful :) Thank you so much for posting this!

(I've previously tried pre-transforming on the image side to do color contrast enhancement, but without success: I take the Sobel filter of an image, and use it to identify regions where I boost contrast. However, since this is a step preceding "rasterization", the results don't align well with character grids.)