...what hope for paint applied in 2ooo BC?
Well, if the paints are inorganic, Mr H, they stand a fair chance of surviving (if they've been kept in a reasonably stable environment that is). I don't know if anyone's actually done any research on this (evidence of paint on megaliths) but it would certainly make an interesting study. Wasn't there a theory a few years ago that the Thornborough Henges may have been coated with gypsum when first built - that being the case it might suggest there was a painting tradition on a megalithic scale in the Neolithic.