I always stamp with Stazon ink. I color with Galaxy Markers or Fabrico markers. If I do those I don't seal the tiles. If I use chalks I do use the matte finish Krylon Fixative spray. I have never personally used colored pencils on the tiles. I have also never tried watercoloring.
My friend Dini makes gorgeous tiles. Here is the link/address for her gallery:
http://pg.photos.yahoo.com/ph/oryswife/my_photos
Most of her stuff is Biblical Impressions but you will get ideas galore. Just scroll down to the album marked "Tiles". If you have any questions for her I can pass them along. She is in Indonesia as a missionary right now.
HTH,