The positive and negative valences are arbitrarily assigned. They are assigned to represent the opponent characteristics of the colors.
the chromatic valence represents the amount of color seen at different wavelengths. For example, at 450 nm the blue valence is approximately -1.0 and the red valence is approximately 0.2. These valences indicate that the perceived color should be blue with a small red component. This color is usually called violet or purple. At 550 nm there is a large yellow and a large green chromatic valence indicating that this color should appear greenish yellow, which is what most people see.
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