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.

Indeed. Which is why I said:
"The weather up here in these harsh wet northern climes would have meant that any stones outdoors here would have to have been painted afresh each season or according to whatever festivity was being celebrated."