“Besides occupying the centre of Leonardo’s painting, Christ’s spatially isolated from the apostles, all of whom are bunched together as they physically touch their neighbors or lean across one another in partial eclipses. Leonardo further highlighted Christ by placing him against a window that opens onto a landscape of clear sky and bluish contours –by giving him, in effect, a halo of sky. The effect is dazzling, even despite the color loss, as the warm tones of Christ’s face, hair, and reddish undergarment advance while the cool blues of the landscape recede: a prime example of Leonardo’s knowledge of the push and pull of colors. For the blue mantle over Christ’s left shoulder Leonardo used ultramarine, which was, along with gold, the brightest and most expensive of all pigments. One fifteenth-century treatise on painting called it ‘a color noble, beautiful, and perfect beyond all other colors.’ A singly ounce could cost as much as eight ducats, more than the annual rent paid on a house by a poor worker in Florence. So expensive was ultramarine (the only known supply came from Afghanistan) that unscrupulous thieves sometimes scraped it from paintings. Because of tis beauty and expense, it was used to color the most prestigious and venerated parts of a painting, most notably the mantle of the Virgin Mary.
The colors of Christ’s reddish undergarment were equally bright and deliberately intensified. Leonardo generally laid his colors on a base coat of lead white spread across the entire wall. For this red garment, however, he covered his white primer with a carbon-based black pigment to create dark foundation, then added vermilion. Vermilion was the most brilliant of all reds, and its appearance on the wall of Santa Maria delle Grazie would be all the more striking because it was a pigment that like ultramarine could not be used in fresco. Vermilion was made from cinnabar, a brick-red mineral that ancient Romans believed came from the blood of dragons crushed to death under the weight of elephants. Like most mineral-based pigments, it was incompatible with lime. Indeed, the layering of five separate coats of paint, carefully manipulated to intensify their values, was something else completely unknown to fresco.”
Source: Leonardo and the Last Supper by Ross King