It is difficult to believe beforehand that we can measure what we perceive. But soon we remember that exists something in drawing called "Linear Perspective" that the Big Masters of the Renaissance have discovered as a peculiar science of seeing-through. The good news is that the colors are not the exception; the scale of values is one example.
What is exactly a scale of values? In art, we call value to the kind of position that a color has in terms of light and darkness. Any color, no matter the pigment - hue, has a value situation. Depending on how dark it is, we can even organize it among other colors in what is so-called the scale of values. In doing so you will have a way that goes from the darkest to the lightest.
However, a true scale as any stairs will be seriously done if each difference between steps has the same distance - otherwise, we risk falling. Can we be precise in making a scale of values? How to be sure that the distance between a pair of colors is the same than the next one? We will be sure if we get into the phenomenon of the interaction of colors. We need to analyze the interaction of three colors in a transversal way so that we are able to see the next ones on each side. Then you have five colors all at the same distance to each other, and you can keep going finding the other two. The distance is defined by your first three colors so the projection on both sides will keep it. If the three first colors are very close to each other or not is your decision. Then the sequence just grows from it.
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