The colors and the letters ( a, b ) indicate substyles (style variations). Some styles, called multi-styles have a, b, c, & d substyles. Play around switching styles in a song and you will quickly see what is going on.

As for how to do a "intro/ verse /chorus /verse /chorus/ bridge/ chorus/ end" song format, I will have to defer to others, since I don't generally use repeating formats. But keep in mind that you can use the menu option "Edit >> Copy From... To..." to copy large portions (any number of bars of a song, including Melody and Soloists.

Hope that helps.