Greens a primary color because your eye has four type of receptors that are sensitive to light, and each to a different range and degree, one of those is essentially green. That's just what nature and chemistry allowed. The first type of receptor, your rods, are only functional when there's very little light, if you ever woken up late at night and been able to make out the room by the light provided by a power button light on an appliance that's your rods working, they're actually green sensitive themselves, or a bit green-blue, so a little green light on an appliance can be enough to see to navigate around a room.

Rods are incidental when it isn't dark though, they're not what lets you see green light normally. That is the M receptor. There is also a S and L receptor, short, medium, long, or commonly blue, green, and red respectively. Only they arne't really red, green, and blue, they are sensitive to a range of colors that peaks out there, you can see that on this graph. But the M receptor doens't see green or yellow or blue-green each. It just goes off like a bell if any of those hit, it doesn't know or care which. Like if there was a line of people walking by and I told Lucy, Megan, and Sally to count them, but told Megan to record anyone who passed by who was 5'4"to 5'8", to tap a little bell each time. Megan does this every time one passes by regardless of if they're 5'4" or 5'10", rings the same bell. I've told Sally to tap a drum every time someone passes by who is 5'0" to 5'6" and Lucy to blow a whistle anytime someone 5'8" to 6'2" passes by. When Lucy blows her whistles and the other two do nothing, I know the person is 5'10"-6'2", 'Red', if Megan rings her bell but neither other does, they 5'6" to 5'8", and Sally's sole drum beat means the person is under 5'4", or blue. If Sally and Megan both sound off, the person is 5'4" to 5'6", blue-green, and Megan and Lucy sounding off would be 5'8" to 10, or yellow. That's essentially the concept of a basic spectrum here, with red as very tall and violet as very short.

Now, replace the line, one person wide, with a big column of troops walking by, and I've told the gals to look at each row and drum, blow or chime once for each row, and to just do it louder if they see a lot of their persons. They also aren't too sure, counting quickly like that, about the people far from the middle of their height range. They have peak accuracy for their own solo sounds but tend to not count the overlap as well and occasionally count one outside their official range. Their accuracy graph as something of a hill, except Lucy, our tall counter, has a tendency to count very short people too from time to time.

This is where we finally get into cyan, yellow, magenta, and white. Anytime a row passes by that they all sound off equally loudly I note down "White", and its brightness or dimness is how loud they are, but the color is ratio of how loud they blow, equal and white. If Megan and Sally both sound off loudly and Lucy does nothing or barely anything, I call that Cyan and it doens't matter to me if the hundred or so people in that row are all 5'5" clones of each other or a 50/50 mix of 5'3" blues and 5'6" greens. All I know are Megan and Sally are making a racket and Lucy is twiddling her thumbs. Ditto when Megan and Lucy go off I don't know if they just counted a whole bunch of people of a single 'yellow' height or a roughly equal mix of red and green.

So Magenta... I know Sally will go off for very short people to medium short, but I also know that Lucy has a habit of sounding off for the people so short Megan doesn't sound off for them on accident nearly as often. I've gotten into the habit of calling these my purples, violet and indigo. So on the occasions that a row passes by where sally is sounding off and Lucy too, but not Megan, I mark those people as very short, purple. I don't really have a sound for all tall and short, no medium, this is an average effect and if Megan is staying silent I know there's no medium heights. So if Lucy and Sally are both chiming I call it purple. And when a row of all tall and short but no medium go by, I still call it purple, or specifically magenta, because for violet and Indigo Lucy does go off but not nearly as loud as Sally, still I view it as a purple color even though I know it isn't quite right, because my notepad only allows me to mark for each row which instruments went off and how loud.

I also can't make good guesses off raw volume because any given row walking by might have 3 guys or 300. Those photon troops are very numerous, whole armies, and I've got wads of checkpoints with clones of the gals each sending going off and clones of me, the 'cone' each writing our reports and running them over to your brain. In reality these individual checkpoints aren't manned by three girls but a thousands of them, 2% are Sallyies, 30% or so Megans, and 60-some% Lucy's. That's why Lucy's occasional purple/short whistles are so hard to simply ignore.

So it isn't that magenta is exactly a fake color, it just how we interpret a racket by Lucy and Sally when Megan does nothing and Lucy seems a little loud for the normal violet shade short folk. Odds are if Lucy didn't have the bad habit of going off for very short people on top of her proper tall range, or if there weren't so many of her compared Megan and especially Sally, we probably wouldn't interpret a mix of red and blue as purple at all but something else entirely or maybe just call it white.

But the upshot of all of this is I can get away with using only troops of three specific heights, 5'1", 5'6", and 5'11", plus or minus an inch, in the right combinations to get the gals to produce any specific mix of drum, bell, and whistle I want, and in some ways better since I only use the colors, or 'heights' that they never screw up on by falsely going off or skipping.

Those heights are my 'primary additive colors'. We can add in troops of those heights and produce any color we wish and brightness we wish. Cyan, Yellow, and Magenta are considered the secondary colors for additive, the colors we can make by a mix of any two primaries we add in.

We also have something called 'subtractive colors'. If my incoming row of troops are a big mix, white, or every sort of height randomly, I can stick people out who come by and intercept people and subtract them from the line. I can produce any color this way by taking my big white row and removing everyone of certain heights. Here I can produce any color by subtracting the people and the primary colors and secondaries flip - cyan, yellow, and magenta are primary subtractive and red, green, blue are secondary subtractive. Lucy, Meg, and Sally don't have a clue, they still count like normal. Just in one case I'm throwing in people of specific heights to adjust color and in the other I'm yanking them out of the line.

Paint mixing is subtractive, ditto ink and so on. Primaries are magenta, yellow, and cyan. A lot of us are use dot it being red, yellow, blue but that's just bad habit, its close enough to right not to be obviously wrong. You can get better spectrum of colors using magenta, yellow, and cyan then red, yellow, blue. Magenta isn't a very commonly natural color in the sense that most things principally reflect a single band of color, the width might vary but it's still a hill, be it a plateau or sharply rising rock. To get magenta you essentially need a color that reflects most visible light then mix in something that absorbs green light. Doubtless if in olden times artists and teachers had access to a wider range of inks and dyes we'd have used the right primary subtractive colors. Subtractive method lets you make a wide range of colors with a lot of different basic shades but those three let you do it with the least material, thus least loss of light and highest contrasts and resolutions.

... and hopefully that helped clarify, if not I'm giving up on the subject, I can't think of a better analogy and the net seems to be surprisingly absent nice pre-existing ones to look up.