Kinda fueled by my recent thread in dev help, but addition of difference would allow for a less costly way to apply a color inversion effect. While mapcolors has this ability and you can cache the result, there are edge cases, typically mobs, that require the icon to be remade. This is performance hit is increased for atoms that utilize several layers of overlays to achieve its appearance because it requires a 'flat' icon to be generated beforehand and only adds to performance degredation.

Could also add and and or as well if possible.
This likely won't get added as most people wouldn't use it but is worth a shot.