Boundaries and hairlines
You will often find that you can intensify colors by making boundaries between them. The simplest boundaries are white and black. If I have a red plum on a blue bowl, and there is a line of white above the plum, it seems like the light on the surface of the plum. But what really happens in the painting is that the whitish line connects the red of the plum and the blue of the bowl, unifies them, and makes a stronger connection. A black line often does the same.
Of course, one doesn’t use actual white or actual black — these are usually too stark. A white which has red or green or blue in it will work better — you have to look and see which one does the most. The same is true of black — it will usually be an off-black, with brown or red or blue in it. And boundaries of more subtle color often work to do even more.
As one tries to reach inner light, one is in effect trying to create a deep kind of unity. This unity can be helped, in part, by mutual embedding. But the problem which needs to be solved occurs, in principle, at the boundaries of each color. Each patch of color has the danger of being too isolated. Where two colors meet, there is an imperfect unity just because the two colors, be being different, create a divide. To bridge this divide, it is helpful in the vast majority of cases to have a third color, much smaller in extent and carefully chosen in color, which forms a link across the boundary. That is why hairlines and boundaries originate.
The general phenomenon goes like this: We have two colors, A and B. Then the unity of A and B together is enhanced by a thin line or zone of a third color C which bears a definite relation to A and B.
We see this rule employed at two different levels: first, in cases where C is a broad swath, or stripe of color, and second, in cases where C is a thin hairline of color. Occasionally we also see a third case in which these two ideas are combined to give a configuration that has five colors: two major colors, A and B, separated by a broad band of boundary color C, and then A is separated from C by a hairline of color D, and B is separated from C by another hairline of color E.
In general the boundary color must be chosen to do the same as any good geometrical boundary does: that is, to both unite and separate the two colors on either side of it. For any two colors A and B, it is possible to choose a C for a good hairline as a separator/unifier of these two colors.
There are two laws of a good hairline, defining the choice of the color C. First, a law of contrast. If we define the three color values (darkness), for A, C, B, with C being the hairline, then the three values must always be different. This gives only three possible schemes for the contrasting values:
|A (first color)||C (hairline)||B (second color)|
Second, there is a law of unity. The hues are chosen in such a way that both A to C and B to C have the following properties. C is similar to A in one respect and C is different from A in some other respect; C is similar to B in the way that it is different from A. C is different from B in the way that it is similar to A. Thus if C and A are similar in hue, and different in value, then C and B will be more similar in value, and complementary in hue.
All these examples show the impact of the boundaries in color: all are similar in the way they work to unite areas, just as geometric boundaries work. Only the thickness of the boundaries is different.