[svieira]

There is nothing on the fibers that is visible under a high powered microscope. Paint (even spray-paint) is quite visibly distinct from the fiber it covers.

[tsimionescu]

When it is relatively fresh, or well protected. Who know what is left after 600 years of handling and sunlight exposure and being saved from a fire.

[svieira]

We have known paintings from the time period - the paint is still visible under a microscope.

[tsimionescu]

Yes, because those paintings were preserved as paintings. And they were not designed in the first place to look like something other than a painting.