Here is the painting the Virgin, the Child and Saint Ann
(circa 1510) by Leonardo da Vinci (1452–1519):
(click to
enlarge)
And here is the same painting after a rather aggressive restoration
using the GIMP:
(click to
enlarge)
To obtain that effect, I first lightened the picture considerably. Then I applied ad hoc curves to the red, green and blue channels (notably the blue, which has suffered considerable loss in the picture) to make the colors more white. After that, I made a copy of the picture and, in that copy, I uniformly colored the hue and saturation parts (leaving the value channel untouched) of certain regions of the paintings, notably the Virgin's robe; then I remixed the copy with the original to attenuate the effect. It is quite evident, if you look in the details, that I was rather sloppy in the coloring. Still, if you don't look too closely, the overall effect is rather satisfactory.