as usual here in the Academic Forum I have to ask the first standrad question:
ST10 with the MP3 or ST without any MP?
If no MP then I would suggest to download the later ST10 file with MP3 included, or, if You are at a UNiversity with maintenacne contract (Solid Edge University version) then ask Your prof to take care about the current MPs
On the other hand, I do not now about any issues when assigning a material to an open part.
So let mes ask following question too: Which material file are You using?
The standard MTL delivered with ST10 or maybe an old one from a former version?