in Thoreau’s Journal:

Think how much the eyes of painters—both artisans & artists—& of the manufacturer of cloth & paper—& the paper stainers—&c are to be educated by these autumnal colors. The stationer’s envelopes may be of very various tints—yet not so various as those of the leaves of a simple tree sometimes—If you want a different shade or tint of a particular color you have only to look further within or without the tree—or the wood. The eye might thus be taught to distinguish color & appreciate a difference of shade or tint.