Magenta is a peculiar colour. We use it all the time in printers along with cyan and yellow.
The weird thing is, you won't find magenta in a rainbow!
Rainbows are spectacular displays of the colour spectrum. Light travels in waves and the length of the wave defines it's colour. The colour with the shortest wavelength that we are able to see is violet and the colour with the longest is red.
Green is somewhere in between so it's wavelength is somewhere between that of violet and that of red. Pick any colour from the rainbow and you can work out it's wavelength from it's position. Or pick any position and there will be a totally unique colour there with it's own wavelength.
So how do primary colours fit into all this? If every colour is unique with it's very own wavelength, how can you make them by just mixing together the three primary colours - red, green and blue?
Well it turns out that primary colours have nothing to do with physics and everything to do with biology. The primary colours exist because of the way our eyes work.
At the back of our eyes there are little cones that collect light. When light falls on one of these cones it sends a message to the brain to let it know.
The more light that falls, the stronger the signal that is sent to the brain. But what about colour? It turns out that there are three types of cone that correspond to red, green and blue. So if you're looking at something blue, it's the blue-sensitive cones that react and tell the brain there's something blue out there and so on.
What happens when you look at something yellow? Yellow is inbetween red and green an the spectrum.
Because it's not quite red and not quite green it excites both the red-sensitive cones and green-sensitive cones but only a bit! The brain gets a signal from both cones and deduces that it must be looking at something in the middle. Something yellow! Correct. If your brain is successfully piecing together the indirect information it's getting from your eyes then it's doing a good job.
The same goes for cyan.
It's halfway between blue and green. So both those cones fire a little and your brain deduces that it's looking at something inbetween.
But how should you brain interpret a signal coming from the red cone and the blue cone but not the green cone? Should it do what it did in the last two cases and assume you are looking at something half way between red and blue? Well red and blue are at opposite ends of the spectrum so the half way mark would be green. That's no good because the thing you're looking at couldn't possibly be green or your green cones would be getting excited. So instead your brain makes up a colour. That colour is magenta.
Have a look at a colour wheel.
A colour wheel is made by taking the two ends of the spectrum and bending the whole thing round into a circle so that the ends meet. These ends have no physical reason to be together. So if you force them together that's where you'll find magenta, the brain's little trick.
In this version I've added a black line. This shows the original spectrum where the wavelength gradually decreases as you go round. And the black dot is where you've got two different wavelengths that don't belong together. That discontinuity is where magenta lives.
It would be remiss of me not to mention the "controversy" around this subject.
This article says magenta isn't a colour.
This one says don't be daft of course it is.
My opinion is that the argument is more about semantics than anything else. It's a bit over-the-top to say that magenta isn't a colour but it's a catchy title and the details are cleared up in the text.
Yes, there are other "non spectrum" colours but magenta is the most striking example and the easiest to explain.
And yes arguably no colours are real from a philosophical point of view because we only ever "see" them in our brains. But the fact is, red, green, blue, yellow, cyan, and infinitely many other colours are all on the spectrum but magenta is not. That's pretty cool.
All that might be redundant now as the original article looks like it's been updated to say pretty much the same thing.
And I'll just tidy up one other loose end. There are actually two sets of primary colours: additive and subtractive. Typically red, green and blue are used as additive primaries and cyan, magenta and yellow are used as subtractive.
Basically, one set is for adding colours and one is for subtracting.
Red, green and blue are for adding. For example, if you start off with black then add red and green you get yellow. If you finally add blue to the mix you get white.
That's why a TV uses red, green and blue pixels. It's a process of adding colours together. Incidentally, that is also why you can't make indigo and violet on a TV or computer screen because it is beyond blue. It's only ever faked.
With the cyan, magenta, yellow set you start with white and take colours away. For example, if you use magenta what you're really doing is taking away green. You can think of magenta as the lack of green! If you follow that with yellow that's like taking way blue. So what you've just done is take away green and blue leaving you with red! So now you know how your printer makes red - with magenta and yellow. What if you then put cyan on top? Cyan is just the absence of red so we'd be left with black.
You can see the relationship between the two groups here. The overlaps of one are just the mains or the other.
You probably learned as a kid that yellow and blue make green. It's actually yellow and cyan you want for the best green. And it's just white minus red minus blue = green. Where "minus red" = cyan and "minus blue" = yellow. Does that make sense?
Finally, I'll reproduce the second optical illusion from Liz Elliott's post. I've recreated it from scratch because the original has a couple of odd glitches in it. Feel free to pinch it Liz.
It's great because it illustrates how magenta is just the absence of green. The first thing that happens is that your eyes adjust to the dots (when you stare at the middle) and they disappear. Then when you take away the things that's already disappeared you're left with it's inverse.
Check out this cool audio illusion.