If the filenames are the same in both places, I believe you could also select all the files with color classes, use the Text Exporter to create a text file that had Tab separated filenames and color ratings, then load that into PM as a Code Replacement file, and then when viewing the backup files in PM, apply a Metadata template where you would select "Evaluate" where Color Class is and put "={filename}=" (without quotes) into the new field.
I confess I am just winging this out without testing it but I think it should work right?