The Missing Color
The answer is, of course, green.
We are a product of our environment - the air, the water, the minerals, the plants, the sunlight. And, just like plants, biological life is an enfoldment or materialized reflection of light. The colors we see in the sky are a product of an octave of light (370Thz - 740Thz) refracting through an oxygen atmosphere of blue air and water molecules which, with dust, refract the light around the color wheel to end at yellow, the color of our sun. Organic life does the same thing, only in reverse. Plants absorb all of the colors of the sky except the frequency band of green, which it reflects by default while emitting oxygen. In this way, plants are an extension of the sky and complete its frequency spectrum. Just as the green leading tone is pulled up to the cyan tonic, so is the green plant's oxygen pulled up to the cyan sky. The two form a standing wave with the yellow sun feeding the plants which in turn feeds the atmosphere (A->B->C on the color octave). Biological life also feeds on plants, but through ingestion and not directly on their oxygen. Thus, the green frequencies plants reflect are absorbed by green cones on our retina weighted toward the center of our visual spectrum as a survival guide. Just as the plants complete the color spectrum of the sky, plants complete the visual spectrum of biological life. A world full of biological life depends on oxygen which depends on plants which depends on the sun and this is all represented in how colors are refracted, absorbed, reflected and perceived by all things concerned.
Green could be considered the invisible 90-degree pivot point. In the color spectrum, its plant life lies between the yellow sun and the cyan sky.