I'm sure you guys have seen the new Celtics version of the ESPN/NBA Playoffs commercial showing Pierce on the rafters discussing the championship banners.  Pretty great. 

Upon reviewing the banners though, I was wondering if anyone knew why the round NBA logo on the banners differentiates from being a solid green to a white circle.  I thought maybe it was just a random coincidence from the 50's and 60's but even the 2008 banner varies from the 80s ones.  See below:

http://upload.wikimedia.org/wikipedia/commons/0/0b/BostonCelticsChampionshipBanners.jpg

Anyone know the reason for this?