by louper
The purple die can be used as any color, so yes, it can be used with 2 yellow dice to make a group of 3 yellow dice.With a 6, you have 6 action points for use on the ramparts. You can spend:
1 action point to move your guard 1 space along the ramparts. Do this up to 6 times.
1 action point to put a cube on any activity you have reached or passed. Do this AT MOST once per activity.
So, you could advance the guard 3 spaces and then put a single cube on activities I, II, and III, provided you have reached activity III's rampart.
The only time you'd waste a die used along the ramparts is if your guard has already made a complete revolution of the city (and therefore can't advance any further). Otherwise, I think you should always spend any excess action points to simply advance him further.