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Pigments and Perception

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1301263.  Wed Oct 31, 2018 6:56 pm Reply with quote

There was a mistake in episode 6 of the P series, "Pictures". A very slight, offhand mistake, but, this being QI where nits are picked with more frequency and zeal than a card - any card - at a magicians' convention, I'm going to point it out.

Sandi made this remark:
"... nor is there such a thing as reddish green."

Now, "what", you may be asking yourself, "is wrong about that?" Because, it very certainly is true that you can't have a single colour that looks like both of two complementary colours. The problem is this:

There is such a thing as reddish green. It has a name: Yellow. Because Red and green are not - contrary to popular misconception - complementary colours.

"But what about the colour wheel? We learned this in school!"

Ah, you see, this is all the fault of the RYB colour model... a model which is not accurate to how human colour vision actually works. This whole topic is a severe pet peeve of mine, so gather 'round while I spin you a yarn of prismatic facts, in detail more than necessary.

Now, there's a whole lot to cover with this, but let's start at the beginning with Newton. "ROYGBIV", a mnemonic virtually every English speaking person is familiar with. Seven colours. Newton is the one responsible for this, and it was not because of any underlying nature of colours or perception, but because he was a biblical numerologist, and believed the number seven held some sort of cosmic importance. It was entirely arbitrary. The rainbow itself isn't made of seven distinct colour bands, it's a smooth spectrum of frequencies. And human colour vision isn't based on seven. Among the vestiges of this seven-colour rainbow was the RYB colour model, which still persists to some degree, to this day, but should be abandoned, because it gives people an inaccurate comprehension of colours.

As for why red, yellow, and blue are not the actual primary triad, I'll point first to printing. There are two kinds of ways of colouring things. Either using light, or using pigments. Additive or subtractive colour. In printing, you have CMYK: Cyan, Magenta, Yellow, and Key (Black). These are the primary subtractive colours, plus black. The subtractive primaries are the complements of the additive primaries. Notice how yellow is in there and not with red or blue? This is because the structures in the human eye that detect colour detect it in red, green, and blue. Green. Not Yellow. Yellow is what you get when your eye detects a combination of red and green, or detects a frequency between the two, since human colour vision detects frequencies along a bell-like curve, and a frequency between red and green will set off both the red-detecting structure in the eye and the green-detecting structure in the eye, and so, you see yellow.

And this is all made even more apparent by the fact that colour vision deficiency is in terms of red, green, and blue. The well known "red-green colourblindness" is because yellow isn't one of the primary colour frequencies human eyes detect.

Here is a colour wheel using the RYB model, and I have been generous with the saturation of the green. It should be gray, but for the sake of showing the "between" colour on the full wheel, I have the cyan-green that is between blue and yellow on that side of the wheel:

Compared with the true colour wheel, using the RGB model:

Notice how the RYB model has no cyan? It's completely missing, and yet, it is a colour. And how the green seems to be more subdued? That's because it isn't true green. It's the colour between cyan and green. It's mint: a tertiary colour. And, worth clarification, is that either set of primary triads are also secondary colours, depending on if it's additive or subtractive, but the tertiary colours are always tertiary.

So, to recap:
The three additive primary colours (of additive colour) are red, green, and blue. The respective complements of each are cyan, magenta, and yellow. Red and cyan are complements. Green and magenta are complements. And blue and yellow are complements. Red and green, because they are not complements, can be combined to give you yellow.

Also, in practice, the RYB colour model produces muddied colours. It's certainly a model you can use, and has been used in the past, but it produces muddier colours, namely when used as pigment primaries, and it produces a more muted green, if not simply gray, or a brownish colour. The other issue of using the additive primaries as pigment primaries is that if you mix red paint and blue paint, you get a dark magenta, also known as purple. That's right, purple isn't a secondary colour. It's a darker shade of one. The common understanding of colour theory is not accurate. Magenta is the secondary colour, and purple is magenta that is darkened. Also, the sky isn't true blue. It's more somewhere between blue and cyan, i.e. azure. But that's sort of moot, because it's arguable that that's a variation of blue, but it's also arguably a variation of cyan.

Another thing I bemoan about Newton's legacy with regards to colour is that people think orange is a secondary colour, when it's a tertiary colour. It's between a primary and a secondary, whether you're using additive or subtractive colours. I also loathe that magenta and cyan are not thought of as primaries or secondaries. But they are fundamental colours exactly as much as yellow is, unlike purple or orange.

Now, what to do about the mnemonic? Well, firstly, it should start at the only nonarbitrary point on the colour wheel, i.e. the point at which human colour vision of the electromagnetic spectrum begins and ends: magenta

And, to include orange, you'll have to include the other tertiary colours as well, and we get this:

Mr. Roy C. G. McAbv

Magenta, Rose, Red, Orange, Yellow, Chartreuse, Green, Mint, Cyan, Azure, Blue, and Violet. It's longer, but works as a name to remember.

If you exclude the tertiaries, you get Mr. Ygcb, which isn't as good of a mnemonic, but it'll have to do. As a consolation for that, I'll throw in a bonus mnemonic for how to spell "mnemonic": Memorizing New Expressions Made Of Notably Inventive Components.

Anyway, the point is that red and green are not opposites on the colour wheel, and you can absolutely have a reddish green, and that colour is called yellow. The human eye works in RGB, and not in RYB, and both cyan and magenta are as fundamental as yellow. That's a points penalty to Sandi.


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