I would have to see your footage to give a more detailed response, but to give a loose response to your question about seperating the purple pixels in the face from those in the clothes: Track the head and apply the track information to a white solid, positioned and matted to only cover the head. Duplicate the original footage, and apply your tracked solid as the alpha of one, and the invert alpha of the other. Then you will be able to deal with the purple dots in the face seperately from those in the clothes. Does this help?
d.sprowls is pointing you in the right direction. For some more detail, here are two videos that I made that show a couple of different ways to isolate and apply an effect to only a person's face:
These should give you some ideas and get you going.
I am a complete novice so I'll take a look at this and see if I can do the steps like you say. Keying is a nightmare!