Hello,
the proxy file does not contain materials in itself, it just knows the amount and position of stacked material tags.
If you had an object with 7 material tags and none of them contained a duplicate material, copying those tags from the object on the proxy would create the exact render output as the object.
However, if the object contains some duplicate materials, this will mess with the allocated material slots and those duplicate tags will need to be manually removed.
Thanks,
Jan