For many years now, I have found it puzzling that indigo is not represented here:
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Nor here:
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Outside of the primary and secondary colors, there are countless colors, including indigo.
Why do you suppose that indigo gets special treatment?
I have pondered the question myself.
My best guess is that Newton was either receiving instructions from on high or receiving an unction/epiphany concerning their being Seven Colors, in such cases we have a Knowing without having the knowledge to ascertain and declare how or why an assertion is so;
The Seventh Color [Magenta] is Realized rather than Seen plainly, I.e., when we are beholding Cyan (blue) we are actually seeing the presence of Blue and Green overlapping, likewise when we are beholding Yellow we are actually seeing the presence of Red and Green overlapping, wherefore when we are beholding (a luminous) Violet we are actually seeing the presence of Magenta and Blue overlapping, and when we are beholding (a Pigmentation of) Red we are actually seeing the presence of Magenta and Yellow overlapping, ergo I believe Magenta is the Seventh (veiled) Color that eluded Newton’s detection.
But what had puzzled me for the longest, was how come we do not behold Magenta in a Rainbow or in the refraction of a Prism?
The answer to this question also gives us a clue as to how the refraction process occurs, I.e., when we behold a Rainbow it is actually a full Circle, and the Colors produced by a Prism and in a Rainbow are arranged in the same sequence as we would expect to find them on a Color Wheel, in short, the Colors are arcing away from the viewer with Green being closest and Red and Violet being furthest blending into obscure darkness, wherefore I am not entirely sure that Magenta is not present, the visibility at either end of the spectrum is very poor.
The next time you see a Red Rose, zoom in on the pedals, and you will see Magenta with hints of Violet, zoom back out again and all you see is Red.