Well, even layered you’d get the point. They don’t mix like paints do, but we learned about yellow and blue makes green via crayons as a kid. I just tried it right now, and it does make that effect, even though it’s not so much a mix as a layering of colors. Red+yellow+blue “primary” colors makes an ugly pukey color that is somewhat brown.
I figured you did after all that other stuff. Just throwing it out there.
Using only two you get to the middle of the wheel but the values are too low, or high ,depending on what number system you’re using to get black so …grey… devalued black
Using three in theory is black, and it’s darn close with good paint but yeah…
Sometimes I just like to share for onlookers.
Just like Italian, only more so: they call one blu, the other azzurro, the third one celeste.
It the rainbow was designed with the only purpose to look pretty to HUMAN eyes, then yes.
But as it is a simple matter of physics with light interacting with small particles of (usually) water, it has no reason to adapt to our 3-color-sensor eyes.
P.S.
Contrary to common folklore, a rainbow does not have 7 colors.
Even to mere humans with healthy eyes, the number of distinct “colors” in a rainbow varies from about 5 up to 10 distinct bands.
In reality, of course, the rainbow is a smooth continuous spread of frequencies of light, all the way from near infrared through visible light and deep into ultraviolet.
In a way, it’s like trying to classify all sounds in terms of musical notes. Sounds can be any pitch and don’t necessarily sort into the seven notes (plus accidentals) of Western music. Hell, even the notes don’t fit perfectly; that’s why they invented tempering.
Anyway, unless they work with colors professionally, Real Men[sup]TM[/sup] don’t recognize any colors not in a Crayon box. The eight color box.
More Newton stuff, including part of his notes Of Musick:
https://doi.org/10.1179/030801806X143268
There is some reasoning there about how an octave should first be divided into a fifth and a fourth, and so on, until it is broken up into 3 major tones, 2 minor tones, and 2 semitones [so, total = 7], which should be arranged into a mode.
NB also the quote re Opticks where he suggests that the “thicknesses of the Air between the Glasses there, where the Rings are successively made by the limits of the seven Colours” [more or less proportional to the wavelength, if I am not mistaken] is more like 2^{2/3} than a 2:1 ratio, seven Colours or not.
When Newton started writing his Opticks he clearly thought that there were five colors – some sections of the book talk about the Five Colors. Later on, when he saw the analogy between musical octaves and the color spectrum, he decided that there were seven colors, to coincide with the seven tones (do, re, mi, fa, sol, la, and ti – the next “do” belongs to the next octave). I suspect he got the idea from his observations of “Newton’s rings” – interference between a spherical glass piece and a flat glass piece, which give successive orders of spectra as you move outwards from the center, just as the keys of a piano give you successive octaves as you move up the keys.
Having once conceived of the analogy, he then pushed it further. There are definite wavelengths associated with the musical tones. Newton didn’t believe in the wave theory of light, but his work on Newton’s rings showed him that there was a definite length associated with different colors. He equated the two situations, and used the relative wavelengths of musical notes dictate the relative wavelengths of his colors. He had to add more to his five of red, yellow, green, blue, and violet. Orange was an easy choice for fitting between red and yellow, but he had to work on what came in between blue and violet, finally settling on indigo
As I noted in an article I wrote on this, there’s not a sharp demarcation between blue and indigo, and in color-coded applications people sometimes act as if there’s a six-toned spectrum. The Universal Code for Resistors, for instance, uses a six-colored rainbow, with no indigo. I think they knew that people would have trouble distinguishing blue from indigo and left it out. Similarly with a great many “rainbow” flags, which only have six colors. (Not all do, of course, and some have tried to be all-inclusive with seven colors).
I’ve often wondered what would our Rainbow Spectrum look like if Newton had added all the half-step intervals, including the five black keys on the piano. We’d need five more color names. People would probably add imitative descriptions, like “tangerine” and the like.
You got it. Birds can indeed see in the ultraviolet spectrum in a way we can’t:
I do wonder what the world would look like if we (or any creature) had 5-6-7 or more cone types. Would it be possible to make glasses like the ones that they make for red-green color blind people to simulate such vision?
Color reproduction with three-color printing or tri-color monitor screens wouldn’t look right. We’d need to build color systems with however many degrees of freedom you have in color receptors. Or else make things that truly reproduced color (as Lippmann photography does).
Here is a relevant question: what, if any, were Newton’s mystical views of the number seven? What did it mean to him, besides the number of notes in a heptatonic musical scale? Seven classical planets seems obvious, and so on, but what, if anything, did he write on the subject?
Since you bring it up, sometimes you get those resistors with old, faded paint and you end up testing them with the multimeter anyway, just to be sure…
On one hand, Newton states that seven notes are enough in a scale because the additional intervals obtained by further subdivisions would be too dissonant and therefore not musically useful. (So, apparently, he was not into avant-garde atonal music.) On the other hand, in his tuning calculations he does indeed consider the division of the octave into 12 parts as well as 53 (like in Turkish music), 612 parts, etc., out of which you pick 7 to form a melodic mode. The seven named colors correspond to the Dorian mode.
I’m wary of saying anything about Newton’s beliefs. Every time I think I know his mind, he surprises me.
I will say this – the fact that he started out talking about five spectral colors tells me that he didn’t have a preconceived notion of a mystical seven. Although I;'ll grant that his analogy with music giving him seven colors could possibly have been cemented by that number seven. I would have thought Newton too rational for this, many years ago, but I’ve since learned that he did have a mystic streak.
In any event, although several websites make the claim that there are seven colors because of the mystic number seven, nothing really substantiates that view. I’ve looked through old records of rainbow theory , and only one account mentions seven colors – an d two of those colors were white and black.
(A good reference is Boyer’s The Rainbow: From Myth to Mathematics. But it’s by no means the only one.)
Well. each of the 7 occupies a slot. Sorta.
This picture shows what I mean- I can see the indigo-
And in a double rainbow I see a kinda weird greenish purplish faint color between the two.
The dark area between the primary and secondary rainbows is known as Alexander’s band, after Alexander of Aphrodiasis
Not after Alexanders Rag Time band? 
I’m wary of saying anything about Newton’s beliefs. Every time I think I know his mind, he surprises me.
I will say this – the fact that he started out talking about five spectral colors tells me that he didn’t have a preconceived notion of a mystical seven. Although I;'ll grant that his analogy with music giving him seven colors could possibly have been cemented by that number seven. I would have thought Newton too rational for this, many years ago, but I’ve since learned that he did have a mystic streak.
In any event, although several websites make the claim that there are seven colors because of the mystic number seven, nothing really substantiates that view. I’ve looked through old records of rainbow theory , and only one account mentions seven colors – an d two of those colors were white and black.
(A good reference is Boyer’s The Rainbow: From Myth to Mathematics. But it’s by no means the only one.)
This pun is so painfully obvious that I refuse to make it.