“Mix a rose madder with white, let us say, and you get a pink, quite different from the original madder, and the result is a surface color instead of a transparent one, a color you look on instead of into. One does not paint long out of doors before it becomes apparent that a green tree has a lot of red in it. You may not see the red because your eye is blinded by the strong green, but it is there never the less. So if you mix a red with the green you get a sort of mud, each color killing the other. But by the other method. when the green is dry and a rose madder glazed over it you are apt to get what is wanted, and have a richness and glow of one color shining through the other, not to be had by mixing.”

Letter to F.W Weber (1950); as quoted in Maxfield Parrish by Coy Ludwig (1997)
Context: It is generally admitted that the most beautiful qualities of a color are in its transparent state, applied over a white ground with the light shining through the color. A modern Kodachrome is a delight when held up to the light with color luminous like stained glass. So many ask what is meant by transparent color, as though it were some special make. Most all color an artist uses is transparent: only a few are opaque, such as vermillion, cerulean blue, emerald green, the ochres and most yellows, etc. Colors are applied just as they come from the tube, the original purity and quality is never lost: a purple is pure rose madder glowing through a glaze of pure blue over glaze, or vice versa, the quality of each is never vitiated by mixing them together. Mix a rose madder with white, let us say, and you get a pink, quite different from the original madder, and the result is a surface color instead of a transparent one, a color you look on instead of into. One does not paint long out of doors before it becomes apparent that a green tree has a lot of red in it. You may not see the red because your eye is blinded by the strong green, but it is there never the less. So if you mix a red with the green you get a sort of mud, each color killing the other. But by the other method. when the green is dry and a rose madder glazed over it you are apt to get what is wanted, and have a richness and glow of one color shining through the other, not to be had by mixing. Imagine a Rembrandt if his magic browns were mixed together instead of glazed. The result would be a kind of chocolate. Then too, by this method of keeping colors by themselves some can be used which are taboo in mixtures.