I am reading about, and reproducing some Pompeii frescos. The extremely dark reds and blue/black backgrounds (in the so called "fourth period") have been puzzling. Did they paint the darks last? Apparently, the technique was to paint the figures, cut and lightly scrape around them, and then fresh intonaco/pigment and then polish out the whole thing. This explains to me how they could get the inonaco to accept so much pigment so late in the painting. I think they put beeswax on the figures before the new intonaco - this is my own theory. After the whole had cured, the new intonaco was also waxed and then the whole thing was highly polished.