They may have added one of the several reputation modifications. If you do a search you will find a few that may answer your question...
If you find this is not the case, you may want to see if you need to change a setting:
http://www.vbulletin.com/docs/html/reputation
The default is the following:
Red = reputation less than 0
Dark (grey) = reputation 0
Dark green = reputation more than 0
For each 100 positive rep points you get an additional green image and a new reputation title. When you reach 500 it takes 200 additional points to get a bright green.