Not all of the Mouseketeers were equal. Originally, the group was divided into three groups to better manage the children, who had limitations placed on their availability by California law. For example, the Red Team could be filming, the Blue Team could be in classes and the White Team could be on break. It soon became apparent, however, that some kids were more popular than others (Annette) or more pleasant to be around (Annette) or easier to deal with (Annette). This meant that the teams began to signify a mouseketeer's status on the show.
The Red Team became the "It" kids. These were the most popular, talented Mouseketeers who were always front and center. In this group were obviously Annette and Darlene, and also Bobby, Cubby, Lonnie and Tommy.
The Blue Team was the second tier group. They might be used in a dance routine, but never in the front and could fill in for a Red Team member from time to time.
The White Team was the third tier. They were often used onscreen only as spectators or stand-ins. If a grocery store or local parade had a "Meet the Mouseketeers!" promotion, a White Team member was most likely who might appear. (Though the Red Team members were most likely who one might encounter at DISNEYLAND appearances.)
In the final years of the show, the cast was mostly reduced to just Red and Blue Team Members. As production of the show became a well oiled machine, use of the lower tier Mouseketeers became unnecessary.