Under the present black paint, we are told that the background was originally "blue".
The Lady with an Ermine
|(a) - on the left, the blue is invented by me, based on the blues used
by Hans Holbein the Younger (100 years later) and Baldovinetti (20 years
(b) - on the right, the blue is the one in the sky of the Louvre Virgin and Child with St Anne. The Louvre painting is covered with nicotine-coloured varnish, but underneath, Leonardo used the same palette of rusty red, subtle emerald green, gold, cream, and sky blue
|(c) - on the left, the blue is the one in the sky of Leda and the Swan.
This too is covered with darkened varnish but underneath is the same palette
of colours, with this paler, brighter blue peeping through the varnish
over the sky.
(d) - on the right, the blue is the one in the sky of the Last Supper, painted at much the same time as this portrait. Again, the palette is the same, except that the sky is paler still.
Which of these blues seems most likely to have been used by Leonardo to you? Beware your conditioning from later blue background portraits and from the darkened varnish that has covered Leonardo's paintings for centuries. Which version do you want to choose? Which version gives the strongest sense of three-dimensional reality?