Brightly coloured rock art would be lovely wouldn't it. Brings to mind some of those lovely interpretations by the likes of Hob and Kosmik Ken

I'm pretty sure that we are all in agreement that there was a discriminatory processs when it came to selecting the stones to use on a monument but as to whether that covers permiability to paint I ain't so sure. I guess you've got to work with your local raw materials, your local geology. I guess you can only whitewash a stone if you have an available source of raw material for the whitewash. I don't know if there is any evidence of prehistoric lime production - the main ingredient in whitewash.
You mention granite, granite often contains crystals of quartz and feldspar that would catch the sun and perhaps radiate ..or am I going too far with that one?
Many circles contained scatters of quartz chippings or light quartz sands which have been interpretted as ritual deposits.
So I suppose our ancestors were selecting materials that would make the monument stand-out in the landscape. It would be nice to suppose that any post settings within the monument would carry decorated or painted timber structures.
I guess we should also mention ditches, even if ditches we not painted white, there is evidence from various henges that many ditches were maintained and periodically re-dug. A nice fresh ditch would certainly stand-out in the landscape.