The head of the Venus Callipyge outside the Pantheon in Stourhead, Wiltshire, painted in acrylics from a photo (most photos I take now are composed with a painting in mind).  The ever-forgiving acrylics allow for repeated alterations. Blending of a sort was possible with glazing medium and working quickly with wet paint, but tones could also be built up by a kind of scumbling as the paint dried. There was a surprising amount of colour in the grey stone.