The idea is simple :
Add a NBT tag to dropped slimeballs that indicates what color the slime it was dropped from is,
The application of this tag would be toggleable from the config file (so if you aren't using the colored slimeballs feature the slimeballs can still stack),
Then using something like a CIT texture pack you can apply the colored textures based on what color is in the color tag. (I'm not certain that CIT supports this but it does support similar functionality)

This should avoid nearly any incompatibilities with other mods while providing a nice visual improvement that wouldn't even prevent vanilla clients from playing.