Is the Mona Lisa red-haired?

The other day I came across this high-resolution picture of the Mona Lisa. Since the file is very heavy, I won't upload it here. If you zoom in, you will notice that the sitter's hair is reddish, especially on the left side of the painting. 

 

Is this the original colouring of the painting or is the reddish hue due to conservation treatments and cleaning? 

Probably the answer to this question also lies in the identity of the woman portrayed. As you know, the official theory is that she is Lisa Gherardini, wife of wealthy Florentine silk merchant Francesco del Giocondo. The painting is thought to have been commissioned for their new home, and to celebrate the birth of their second son, Andrea. The Italian name for the painting, La Gioconda, means 'jocund' ('happy' or 'jovial') or, literally, 'the jocund one', a pun on the feminine form of Lisa's married name, Giocondo. In French, the title La Joconde has the same meaning.

The other theory considered by art historians concerns Isabella d'Este. Isabella d'Este (1474–1539) was Margravine of Mantua and the most famous patron of the arts of her time. Leonardo was her sister Beatrice d'Este's court painter in the Duchy of Milan. In 1499, after the expulsion of the Sforza (his employers), Leonardo fled to the court of Isabella d'Este. Over a period of three months, Leonardo made several portrait drawings of Isabella (documented by letters). One of these drawings, a profile drawing, is preserved in the Louvre and shows similarities.

From the subsequent years 1501 to 1506, several letters survive in which Isabella—directly and through agents—pursued Leonardo with demands for the promised execution of the (oil) portrait (and her agents promised or also confirmed Leonardo's commencement). The Mona Lisa falls precisely within this period. The hierarchical society of the Renaissance makes the portrait of an upper-class noblewoman more likely than the wife of a modestly merchant.

Other unofficial theories refer to Cecilia Gallerani (the famous Lady with an Ermine),  Costanza d'Avalos, Duchess of Francavilla, Caterina Sforza and Leonardo himself. 

Lady with an Ermine

La dama dei gelsomini, presumed portrait of Caterina Sforza by Lorenzo di Credi

Another interesting theory refers to Isabella of Aragon. This presupposes that the painting took place in the 1490s, during da Vinci's Milanese period, but the painting has officially been dated as later. However, according to Renaissance historian Maike Vogt-Luerssen, not only the sitter is Isabella of Aragon, but she and Leonardo got married after the death of Isabella's husband, Gian Galeazzo Sforza. They had five children (two sons and three daughters). If you scroll down this page you'll find many painting potraying Isabella of Aragon (at least according to Vogt-Lüerssen) and, at the bottom, several books and articles (in English and in German) about the theory Isabella of Aragon/Mona Lisa. 

Portrait of a Lady, by Bernardino Luini (presumed portrait of Isabella of Aragon)

 

Also, this is a portrait by Raffaello Sanzio, allegedly depicting Isabella of Aragon as Mona Lisa.

 

A possible evidence of the sitter being red-haired is this copy of the Mona Lisa, commonly attributed to Leonardo's co-worker Gian Giacomo Caprotti da Oreno, better known as Salaì. Here the red colour is more obvious.