It isn't so much that colors in paintings such as this fade, as it is that the varnish becomes opaque and occludes the colors beneath. Environmental factors, du

It isn't so much that colors in paintings such as this fade, as it is that the varnish becomes opaque and occludes the colors beneath. Environmental factors, dust, and such contribute to how you see the colors, but they haven't gone away. Much of the work of conservation is actually removing the varnish and re-varnishing it. They make photographs like this to help determine -how much- varnish should be removed, and to hedge against damage in the process. Often they must re-apply colors when the removal causes flakes or disturbs unknown faults in the structure of the painting.