Giorgio Vasari, in his book Lives of the, Sculptors, Architects ( Most Eminent Painters, published in 1550, describes the Mona Lisa:
In this head, whoever wished to see how closely art could imitate nature, was able to comprehend it with ease; for in it were counterfeited all the minutenesses that with subtlety are able to be painted, seeing that the eyes had that lustre and watery sheen which are always seen in life, and around them were all those rosy and pearly tints, as well as the lashes, which cannot be represented without the greatest subtlety. The eyebrows, through his having shown the manner in which the hairs spring from the flesh, here more close and here more scanty, and curve according to the pores of the skin, could not be more natural. The nose, with its beautiful nostrils, rosy and tender, appeared to be alive. The mouth, with its opening, and with its ends united by the red of the lips to the flesh-tints of the face, seemed, in truth, to be not colours but flesh. In the pit of the throat, if one gazed upon it intently, could be seen the beating of the pulse. And, indeed, it may be said that it was painted in such a manner as to make every valiant craftsman, be he who he may, tremble and lose heart. He made use, also, of this device: Mona Lisa being very beautiful, he always employed, while he was painting her portrait, persons to play or sing, and jesters, who might make her remain merry, in order to take away that melancholy which painters are often wont to give to the portraits that they paint. And in this work of Leonardo's there was a smile so pleasing, that it was a thing more divine than human to behold; and it was held to be something marvellous, since the reality was not more alive.
So, let's look at the always enigmatic Mona Lisa, as she is today. At least, as close as the computer monitor can reproduce the color.
Now, lets look at one possible version of how she would look with the varnish removed and the color restored.
So, would it be worth it to strip off the varnish? Do a complete cleaning, as was done with the ceiling of the Sistine Chapel (and, boy, what an uproar that caused!)? The painting has already been thru a lot, besides the toll of 500 years. There have been past cleanings, revarnishings, touch ups (in 1809, 1913). In 1956, a vandal threw acid on the lower half of the painting, which required years of restoration. Later that year, another vandal threw a rock at it! This chipped off a speck of pigment, which was painted over during the restoration. Now, Mona Lisa is behind bullet-proof glass.
The Louvre absolutely refuses to consider any kind of restoration work, now or in the future. Would you dare try to work on a painting valued at more than half a billion dollars? My hands start shaking at the very thought.