I read somewhere (I think on Mesa's site) that the color codes are just batch indicators from Mesa, and change often, so they aren't really "standardized" in the heads. It just depends on what batch they are using when they fill the heads they are building. They aren't an indication of biasing, since Mesas are all fixed bias from the factory. They just color code them so you can tell which are matched pairs incase you get mixed sets for some reason.
-AJH