As it stands, you hover over thumb up and it is green. If you hover over thumbs down it is red.
If you've already assigned a decision to a post it should be white for thumbs up or black for thumbs down.
Sometimes I'm reading the GF and a post is so awful I might click thumbs down 7 or 8 times in a week having already assigned it a vote. In rare instances I've already assigned a post a thumbs up... so it should work both ways.