The colour is used by some manufacturers to denote the hardness but the blackones are sold by people like Rimmers simply becasue they don't look as c**p as red ones AFAIK, and I asked Deflex themselves about it.
I've got them in both V8s and they aork great, I removed the Panhard rod ones form Blue after 18 months just to check and they were still mint.
I'm finding teh ride on the LSE a but crashy though so i may go back to rubber, it's also worth noting that Polybushes will limit front end articulation.