Some people who supply poly bushes do use different colours to denote hardness, it's only a colour that is added.
It is possible to get the bushes in ANY colour from certain people though, it is also possible to beef up standard bushes using hard grade poly where any voids exist such as rear diff hanger.
My mate is supplying someone with seriously beefed up engine and gearbox mounts, by filling the void with a shore 90 poly, this has excellent tear/shear characteristics, and the feedback word of "awsome" is most used.
They guy who is having the mounts beefed up wanted yellow bushes so a batch are being done in yellow to suit all his suspension powder coated bits.
Powerflex are seriously overpriced, for the materials used, there is no reason technically or otherwise why for example flo flex, superflex and similar will be any different material wise, my mate has shit loads of the stuff delivered and the company he uses do research into any "unique" properties that some bushes might have, not found any difference from any maker yet from one to another.
It is even possible to have multi coloured bushes, but they are not for structural use, more just aesthetic as have to be cured seperate(colour wise) so not strong just for show.
tabetha