As I said in the first 2 replies to the OP!
Actually the definition of a primary color is a color that can’t be mixed from other colors.
Cyan and magenta displace red and blue in modern color theory with a subtractive system as primary colors for this reason.
This came from Munsell, the father of modern color theory , and came long before printing adopted it.
Here’s an article explaining mixing from magenta.
You’ll see that red can be made from magenta and yellow.
Blue can be made from magenta and cyan.
Good luck mixing magenta from other colors.
Of course, just answering the question as it was posed. Which I thought assumed limitations of human perception and presumed 7 colors, 3 being primary. The whole concept of primary would mean every color in actual existence ( not blended in a way we perceive and define as individuals) if we weren’t presuming human limitations.
If you blend all at once yes.
Rainbow is the result of seperating white light
The seperating is incomplete, where it blends at the " borders". You get secondary colors.
If you vary the blend, you will perceive different colors.
You could make the statement that there is always “blending” taking place in your brain, even with pure spectral colors.
One should not oversimplify real rainbows, either; they will not look exactly like a prismatic spectrum in a darkened room.
Yes, Littleman, you answered the question as it was posed. The problem is that your answer was completely wrong.
While technically correct the infinite color comments are sort of akin to if someone asked how is an all natural cake made and we answered that it can’t be…because cake is a human construct.
In this case we have to assume that individual colors exist as they are commonly experienced.
As you said it shifts from one end of the visible spectrum to the other smoothly, which we see as x amount of colors with x amount of blends.
Only if we frame it from non human perception.
Or perhaps a better example
Why does the sun rise at 6:30am tomorrow?
And we answered explaining that it doesn’t rise and explained Earth’s orbit and also 6:30 doesnt actually exist…we made it up.
Also the very existince of named colors is just made up.
White especially since it’s the whole visible spectrum of light overlapping. It’s one big blend.
When blue and yellow ( also just made up names) overlap we see green.
Rainbows don’t have individual bands of one pure wavelength
They are one big blend,that doesn’t overlap entirely, that we divide into named colors.
If we name colors then we get more than just the three we use to blend a perception of all others we also get some of the other named blends.
Blue and yellow light mixed actually make white, not green.
I’m sure you’re correct. Additive color mixing is not something I can do off the top of my head but still … It’s aside from the point i was trying to make.
But thank you for the correction there.
I thought you needed the red as well for white and green was still blue and yellow just not in equal proportion?
Nevermind, RGB emphasis on the g for typical additive mixing…gotcha
Thanks
Yellow is red and green light. So you have red + green + blue if you’re mixing yellow light with blue light.
The way I remember what colors RGB mixes make is to pair them up with CMY subtractive colors, with RGB corresponding to CMY as complements respectively. If you mix R&G together, you get the complement of B, which is Y. If you mix R&B together, you get the complement of G, which is magenta. Mix G and B, you get the complement of R, cyan. And R+G+B = white.
And mixing the subtractive colors works the same way. C+M = complement of Y, or B. C+Y = complement of M, or G. M +Y = complement of C, or R. And C+M+Y = black. Well, at least in theory. It’s not absolutely perfect with pigments, so C+M+Y is more likely to be a very dark brown, hence one of the reasons there is also a K (black) in print.
Except when it’s yellow light 
There was that Sharp television that claimed to have an RYGB display; I asked about it on these forums and was told it was totally fake (there were extra subpixels but no yellow backlight); in that case it was indeed red+green light.
Yeah, subtractive I do constantly in my artwork. It’s actually something I’ve written guides on.
Haven’t had much use for additive.
Opaque pigments btw , mixing complementary colors gets you grey rather than black.
Transparents get you closer to black but then since the substrate is typically white and they are transparent it still gets you grey if done equal proportions by value , not volume.
Other wise they get you a desaturated version of the the color that is dominating.
Which implies that the statement “the rainbow contains seven colors” is wrong.
But that’s not what you were talking about – you appear to be objecting to fitting the octave into the entire human visual range, not trying to fit it into the space between , say, red and yellow
So it shouldn’t be difficult to just about fit an octave in there, right? Possibly one of the things that persuaded Newton of the verity of his theory. Kepler accepted his theory of the spacings of the planets on even less evidence.
He apparently asked a lot of people, trying to get consensus for his claim of an obvious color between Blue and Violet. I still suspect him of at least metaphorical arm-twisting. Throwing a spectrum on the wall using a prism is one way, but I could see him using Newton’s Rings or other color phenomena as well.
Yes, yes, yes. You know what I mean! 
I work in photography, so when using Photoshop, it comes up all the time as I’m working in RGB and adjusting curves in the individual channels, so I have to think in additive, even though subtractive is what I grew up learning with crayons and paints and all that.
Yes, I know, which is why I said “in theory” they should add up to black, but in practice, they don’t, because of the physical characteristics of pigments, dyes, the medium to which they are applied, etc.
When I was a kid, I was once coloring in my coloring book, and I didn’t have a black crayon, but there was something I wanted to color black in my coloring book. “Aha! I thought. Black’s what you get when you mix equal amounts of the primary colors*”. So I tried to do just that, and found out that crayons don’t mix at all when you scribble them over each other. You can do that with watercolors, but crayons are wax, and you’ll end up simply putting one color atop the other. if you want to mix crayon colors, you gotta melt them together. Or grind them really fine, or something.
My picture ended up as a multicolored mess.
*Well, maybe I wasn’t think of those exact words, but I knew the concepts.