The printer handling color is making an appeal of the most subtle and delicate nature, vastly more so than is made by the type matter that may form the body of the piece of printing he is embellishing with color.
There are three primary colors--red, yellow, and blue--and three composite colors, which can be formed by mixings of primary colors--green, orange, and violet. It is of importance to the printer to know which of these colors are complementary and which uncomplementary. Complementary colors are those that may be used in close conjunction without one unfavorably affecting the other. This is the secret of complementary, or harmonious, colors: Will they make white if mixed? This means a natural and perfect union of the light rays reflected from the color scheme upon the eye's retina, and so passed along to the sensory nerves--the telegraph line from the physical world to the appreciative brain. It appears that those complementary color schemes which can be perfectly justified are such as reflect light rays nearest like the rays that show us white. Red and green, the two most pronounced and vigorous colors, are complementary. When mixed in the proper proportions they produce white, but this does not mean that they weaken each other when otherwise used; when placed side by side they enhance each other's power and brilliancy by reflection. Their very intimate relation is further shown by the fact that red, by itself, is bordered by a faint halo of green, and green by a tinge of red. Yellow and indigo also make white by mixing, and easily reveal traces of each other when properly manipulated. This interchange between complementary colors is carried still further: The shadow of a color does not show the color itself, but the complementary color to which it is most nearly related.
There is a curious law of optical mixture to deal with--that tendency of the eye to unify the color scheme which changes colors when used in combination upon a piece of printing or upon a canvas. This sometimes so changes the expected effect of a color scheme that has been carefully studied as to render it inadvisable to use it. It is generally found that optical mixture verifies the taste and judgment of the colorist who has been faithful to the complementary color laws, and helps him to a harmony, rather than condemns his work. Optical mixture is too nearly a mere name for a manifestation of the relation of complementary colors to trouble the printer, though a consciousness of it and its effect may at times aid him in producing some delicate effects.