about:config settings:
good_rgb_color - default: #33ff33 (green)
bad_rgb_color - default: #ff3333 (red)
font_rgb_color - default: #000000 (black)

Updates:
5-Dec-2007 - Added legend for colors
5-Dec-2007 - Added message while fetching stats
7-Dec-2007 - Moved color legend to bottom of page
30-Sep-2008 - Updated to work with Yahoo 2008 changes

Example screenshots...

Tampa has the 4th best pass D, so it shows up as red. Holding the cursor over the team shows some details about Tampa's pass D:

Fred Jackson has a marvelous matchup against the worst rushing D in the league:

Should color the opposing team on just about any page. Here's the add player page:

Here's the roster page with the 'Opponents' tab clicked at the top. Great for playoff planning...