The reader may ask, since all colors can be mixed from the three primary colors, why others are used at all. Some artists have indeed taken a pride in using only these three, but quite apart from the fact that it is impossible to maintain pigments of the three primary colors in a condition of absolute purity it is very tedious to be forever mixing colors, and other technical difficulties arise as well.
First let us see how another color is derived from two pigments - for instance, green from blue and yellow. The method is the same with dry pigments and those already mixed with a binder.
The grains of color, each perhaps a hundredth or a thousandth of a millimeter in size, cannot be distinguished individually by eye; they combine on the retina in a unified color effect, or, more precisely, not they but the minute blue and yellow particles of reflected light rays. A pervasive green pigment, chrome green, for instance, is different. It does not reflect two different light waves, but one single green one.
In practice it is almost impossible to copy a definite color tone by mixing, so there should always be a series of colors ready in the box, even as there is in the mind, which are permanently there as original colors in the identical tone. These paints enable the painter to see the colors in his imagination as he works, just as the notes of the well-tempered piano live in the imagination of the musician, and from them he can imagine all the delicate nuances and intermediate colors and mix them in practice. Added to this, a larger choice of paints makes him able to deal more easily with all kinds of technical difficulties.
It is almost impossible to vary the tone value of individual paints. to lighter or darker than the original simply by mixing with white or black pigments. White makes the brilliance of pure pigments chalky, and black often has a dirty effect. With yellow it gives a smudgy green, with red an unattractive brown, and with orange also a brown. None of these colors has any brilliance.
There is no need to give an elaborate explanation of the properties of light absorption of the white and black pigments. A practical test is more to the point. Take three tones of alizarin red
light, medium, and dark - and paint a patch of each. Then beneath the dark tone paint a patch of medium tone mixed with black to correspond to the original tone, and beneath the light tone a patch of medium mixed with white.
You will quickly see by the contrasting results how desirable it is to have every possible tone of the important pigments in your paint box. Most synthetic paints are produced in two or three tones. They are not mixed, but the pigment is treated physically or chemically during manufacture. Even so the tones are inadequate to reproduce all the variations of tone in nature.
To remedy this the transparent properties of pure, bright pigments can be utilized. A transparent layer in the complementary color over a dull colored underpainting will provide the rich glowing darks which gave the mysterious depth and glow to the paintings of the old masters.
Whether or not to work with glazes is today an artistic question. Since the Impressionist period it has been rather out of fashion for technical reasons: pictures begun directly from nature, and if possible completed at one sitting, could not be built up on an underpainting, which has to dry all day, while its glazing requires repeated rest periods. Painters and art lovers, too, were enamored of the new effect of flat, opaque colors. However, that is no reason why glazing and its color effects should not be used today, when most pictures are again painted in the studio, and only the sketch is painted direct from the natural model.
At any rate, it would be a pity to be as narrow minded as lenback, who is said to have turned from the work of a rival with the words, "I believe the swine still uses glazes!"