If we compare the Mona Lisa in the Louvre with the copy in the Prado (painted in Leonardo’s workshop by one of his pupils), we can better understand certain details in the original that have become obscured by the layers of oxidized varnish:

• Leonardo created a “sfumato” effect and blurred outlines through his masterful use of atmospheric perspective (Mona Lisa),
• but his pupil, unable to depict reality with his master’s expert subtlety, made every detail distinct (Copy in the Prado),
• The copy in the Prado, restored in 2012, gives us a better idea of the original colors of the Mona Lisa in the Louvre.