FWIW, on the Grand Banks owners forum, which uses what appears to be the same basic formatting software as this forum, threads with unread posts in them are marked with blue "boxes." You can check this out at http://www.grandbanksowners.com
and then open the "GB Owners Board" under "Resources" to see if the blue is a good differentiator for your color perception.
But I would think the highlight color of the box could be set to whatever the forum administrator wants it to be. Assuming this is the case, your suggestion sounds like a good one to me.