As suggested at http://forums.musicbrainz.org/viewtopic.php?pid=19007#p19007 in the new interface it's not possible to distinguish between

1) tags in my files for which MB has no data
2) tags in my files that have matched MB metadata

Both are black. It'd be nice to have some visual indicator so that you can tell #1 is the case (and thus users can add metadata for these fields into MB). If we're going with colours, I'd suggest gray for #1, although not sure if this is great for colour-blind people. Another option is to add some kind of little icon for all tags where the "new" metadata is coming from MB.