Sorry to hear, you're facing this issue. Currently, all plant libraries we resell on our website are from third-party companies and because of huge amount of models we are unable to double check all these models / materials one by one. We've reported some issues with these vegetation models earlier, but honestly I'm sure in which phase it's now as we didn't receive any feedback on it yet.
Looks like, just the first material is always assigned. The hdtrees_8_mentalray_materials.mat
material library seems to include these variations (at least our copy of it - like MentalRay_Berberis_Thunbergii_Green_Carpet_1
) and until it'll be fixed, please assign these "manually".
I would encourage you to contact 3d mentor directly
regarding this problem. Hopefully, once they'll see, there is a real demand from their users, they'll prioritize that. Unfortunately, I'm afraid we can't do much more in this case as we can't modify its content. Thanks for your comprehension.