>> Personally, although I can imagine the gypsum being spread inside the circle for one reason or another,
>> I couldn't imagine the stones themselves being painted white.

Hi BeaverUK

The practice is an age old on in Ireland, so I do not see why it would not have been the case in ancient Britain, too. See 'The White Wife' here - http://www.irishmegaliths.org.uk/derry.htm for an example. It is dying out now, but there is the Three Friars stone row in Kilkenny that are still maintained this way - http://www.megalithomania.com/show/image/320

In the west of Ireland and around Dublin they just used huge quartz blocks where they could instead.