Wouldn't this be much harder if the background had more than one color as well? If that is the case, this new technique may not be applicable to emulators
Well, in the paper they do describe some heuristics for separating figure and ground.
Additionally, a lot of old videogame machines have special built-in routines to place sprites over a background. So if you'd apply the vectorizing algorithm separately for the sprite drawing routine--by not just processing the entire screen after it's rendered, but doing the elements separately--you'd at least not run into the problem that lines from foreground sprites meld with background details.
