What's the evidence that the colors were added later?

[Jorge_Stolfi]

For about 5 years now, it has also been possible to determine when a colour was applied.

Indeed, good to know.  Basically they do C14 dating on the binder used to hold the (inorganic) pigment particles to the substrate, using less than a milligram of paint.

But they are working with oil paintings, where the paint layer is rather thick.   I wonder if they could do anything with the VMS paints, that seem to be much thinner and probably have less binder in their formulas.  They would have to scrape quite  a bit of paint to get the needed amount of binder (10 micrograms) after chemical separation.

Being thinner, the paint layer is also probably more prone to contamination from dirt, mold, and the vellum.

But maybe one day...

All the best, --stolfi