I think there are a lot of improvements that can be made with respect to identifying what messages are new and what messages are old.

First, the feature should be board specific. I may log on today and just go to OkayArtists, but just because I logged in, all the messages in all the boards will be greyed out.

Also if possible the new messages in color should be applied to individual replies within a post. When posts get long, it can be hard to find the new replies.