Although those leaves which have a polished surface are to a great extent of the same colour on the right side and on the reverse, it may happen that the side which is turned towards the atmosphere will have something of the colour of the atmosphere; and it will seem to have more of this colour of the atmosphere in proportion as the eye is nearer to it and sees it more foreshortened. And, without exception the shadows show as darker on the upper side than on the lower, from the contrast offered by the high lights which limit the shadows.
The under side of the leaf, although its colour may be in itself the same as that of the upper side, shows a still finer colour—a colour that is green verging on yellow—and this happens when the leaf is placed between
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.