Mixing convenience
There are some colors that you know you are going to have to use and are a nuisance and time-consuming to mix. Natural looking greens are, perhaps, the most common example. In nature there are a great many greens: yellow green , blue greens, dull greens, pale greens, and so forth. It’s easier to start with a pigment green and doctor it a bit than try to mix a blue and a yellow, or worse yet, keep a dedicated blue and yellow on a limited palette to make a strong green. I used to keep thalo green on my palette (although it stained everything it touched and one artist I know says it’s a “weapon of mass destruction”) just to mix with yellow to get that beautiful yellow green color. It’s easier to keep a sap green or a yellow green already mixed on the palette. It’s much less messy. It’s interesting, also, that no paint manufacturer markets a green that is a blue-yellow mixture, but there are quite a few doctored greens marketed.
Green lends itself to a position on a palette for other reasons and was consider a “primary” color in the Renaissance. It turns out our brain’s system for identifying the color from the information given it by the cones in the retina is based on asking two questions: is it red or green or is it blue or yellow? (There’s third question about white or black.) This is called the “opponent process” theory of colors. See here for more about it. Many artists think the Renaissance artists were on to something, and a palette with red, blue, yellow, and green is often called the “painter’s primary palette.”
It is correct that the cones in our eyes break light energy down into three categories which we turn into the colors red, yellow, and blue (the trichromatic theory ) but the information processing in our brain after that uses four colors. In another “chapter” I discuss the three primary myth.
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