Well now, I have quite accidently found an ancient source for red ocher. I have found a number of firepits lined with sandstone and petrified wood. Sandstone that has been exposed to high temperatures goes through a rather interesting microcrystalline structural change. Mike, if you read this you can contact me at email@example.com and I will send you a sample of the burned sandstone for your evaluation.
Well, you are quite right about heat treated sandstone producing reddish ocher. I have seen this before at camps in far west Texas amidst sandstone outcrops. Heat treated slate will essentially do the same thing although there is little silicate change. It will produce nice blue tints, and of course black more lasting than charcoal. Iron oxide stained mudstone will yeild a nice yellow tint. I have actually made some pretty good looking war paint from all these sources. I do appreciate your e-mail though, as at some point, I'll be glad I have it. Thank You.