Inner light also requires a certain roughness of individual color, a lively variation within the field of a single color. This is similar to the way that roughness works in living structure.
In colors which have light, there are rarely areas of perfectly flat color. Instead, when there are large areas of one color, the inside of these areas vary immensely from point to point so that the overall color is created from the blending or interaction of many slightly different hues.
It is often variation in color which brings the color to life. To make it happen, the colors which contribute to the effect must be members of the same family, colors which are already close. Then we see the composite effect of the varying shades as if it were a single color which has light in it. Geometry also plays an important role in making it work. The variation works exceptionally well when it is half-regular, so that the regular and alternating variation of spots of color together create the shimmering unity.
Both in paintings and in buildings, I have found that this color variation comes about most easily from a process in which you mix the colors on the thing itself, not on the palette. Suppose, for instance, I am painting a surface blue. Suppose that blue is made of blue and white and a touch of green. I keep all three colors on the palette, and mix them on the painting, by taking dabs of one and dabs of the other as I go, gradually making the colored surface create the necessary light. Then I can make sure that the color I put on the place itself always does as much as possible to create the inner light I am looking for. I keep fine-tuning the color as I go, adjusting it, changing it, bringing it back to the right perfect one, according to the overall amount of blue and white and traces of green I have in that area. Thus like a singer who keeps a perfect tone by vibrato, I use the slight variations from point to point to get a more accurate and perfect color balance in the whole.
Sometimes I do the same thing more geometrically. If we have a particular color, say light red, and we mix black with it, we get brownish dark red; and if we mix white with it we get whitish pink-red. These colors may easily become muddy. But suppose that, instead of mixing the black into the red, we put a fine black (blackish red) tracery of points, dots, lines, and curves over the lighter red. This has the same overall effect on the red — that is, as a whole it moves towards brownish dark red — but it leaves it far more brilliant, with sparkle.
In a similar way, fine pink diamonds dispersed throughout the red may again move it towards lighter red, but again without causing the soupy congealed feeling we sometimes get with pink. Again the red moves towards pink, but the white in it sparkles, and the red retains its brilliance.
We may do the same with more than two hues. If we interlace fine yellow lines or arabesques with red, we can achieve a blazing, light-filled orange, far more delicate than we can get merely by mixing red and yellow in the palette, or by painting a homogenous surface orange on the plane.
Thus we get greater brilliance. This is really how, and why, the variation of roughness works: by creating a mixture out of purer colors, so that we keep the purity of the component colors and their interaction. It is vital to realize that the color variation you see will not do anything if it is just mechanical. Mechanical variation will be just that — mechanical. To get a variation which is worthwhile, you have to vary the color during your effort to bring the color (and the whole) to life. If you are paying attention to the feeling which the color generates, and meanwhile varying the color to make this feeling more intense, then you will find that the variation plays a useful role.
Color variation is a version of the geometric property roughness which occurs in all living structure.