I've been experimenting with herbal colorants lately too. Can't speak to the long-term fading, but in prepping the colorant, I've been using a bit of oil in a water bath to pull as much color out of the herbs (spirulina, goldenseal, etc.) as possible before adding them to the oils at trace. This was my latest try. The sandy color is goldenseal and alfalfa. Thus far no color bleed between layers. http://www.soapmakingforum.com/showthread.php?t=50013