Color has nothing to do with hardness on ES bushings. You can get ES in whatever color you want, black is graphite impregnated and less likely to squeak. Color does make a difference in hardness on the GM bushings.
I'm still tempted to try the ES kit. For the $$ you can save a lot if you are going to add/replace all the bushings. The only documented negative I have heard about the ES bushings being too hard is Troy at NAISSO (owner of www.impalaclub.com). And that was secondhand info. Everthing else I've heard is just a repeat of that. Maybe someone should ask him?