From http://bugs.musicbrainz.org/ticket/5824

kyle775 says:

I am colour blind and cannot tell the difference between the small red and green icons used to distinguish matches made between releases and files in Picard Tagger.

It would be nice if this was easier to read, or as suggested by davitof here: http://forums.musicbrainz.org/viewtopic.php?pid=11745#p11745 the icons could be changed some how.