Apparently powerflex is the only manufacturer where colour denotes hardness.
FROM THEIR WEBSITE
Our 4 materials are colour coded to make them easily identifiable. Below explains the shore rating (durometer) and their common uses.
Red 65A – Used for our Universal exhaust mounts and diesel engine mounts.
Yellow 70A – Mainly used for engine mounts, diff mounts and subframe mounts or areas where the bush needs a lot of movement and absorption.
Purple 80A – Our most common material. This is used in most areas of the suspension and provides the right mix between performance and compliance.
Black 95A – Our hardest material. This is used for our Black Series range or on normal road vehicles where the vehicles suspension demands require us to use this hardness of material i.e. in areas of high load.
_
_
__
All the rest just have different coloured bushes with no associated hardness scale.They basically use whatever hardness they deem appropriate for each bush.
In terms of which are best, personally I would just use the best bargain I could find.