--Have you heard of the Prado's Mona Lisa? Basically, that's a copy of the painting in Louvre, attributed to one of Leonardo's students, probably Andrea Salai.
The painting is interesting, because, after a recent restauration, it showed many similarities to the "original", making people realize that the French Mona Lisa looked like that because the original colours were somehow altered by the centuries.
The idea of restoring Mona Lisa was always somehow ostracized by the curators, though: first, because cleaning up the varnish stratifications could damage the work, then because the treatment would alter the original colors anyway. Others comments, though, made people think that Mona Lisa would never be restored because now her "dirty look" is what makes her "an icon", and the reason why visitors love it and "recognize" it.