The question of whether stones and henges may have been 'painted' has been discussed before - both here and on the Stones List. It's an interesting idea but a couple of things need to be remembered. First off, the range of colours available to the Neolithic artist was pretty limited. There was a pretty good white available from calcium carbonate sources and an excellent black that could be obtained from any carbon deposit (soot from camp fires for example). Then there would have been a good range of 'warm' colours from naturally occurring earth deposits. Inorganic blues and greens are far rarer (at least in Britain) and although they are more easily obtained from organic sources (plants and lichens) the colours wouldn't have lasted very long out of doors. In other words, you may have had people back in the Neolithic wearing clothes dyed with a pretty full palette of colours (obtained mostly from organic dyestuffs) but living with a much more restricted pallet (mainly black, white and reds) on external structures such as buildings, stones and totems. Having said that, I might be completely wrong about there being little or no inorganic blues and greens available in this country, there may have been (still may be) sources where those colours were/are found; alternatively, perhaps they were imported, but if they were they would probably have been a rare and expensive commodity (much as was purple until the colour was synthesised).

I can't really see any reason why the stones, at Avebury for example, could not have been painted with inorganic materials but I'm a little worried that no evidence of such paint has ever been found - could be however that any paint that was originally on the stones has completely weathered away over some four millennia or it could be that no-one has actually looked for the evidence.