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In physics, quarks come in one of three color states, usually chosen to be called, "red", "green", and "blue". However, because these are just labels, there are other naming conventions that are used. Murray Gell-Mann used "red", "white", and "blue" because those are the colors of the French flag. Roger Penrose, in his book The Road to Reality, denounces this and states that he uses "red", "yellow", and "blue", as they are the primary colors of paint, rather than the more common "red", "green", and "blue", being the primary colors of light. In almost every source I can find, these seem to be the conventions that people typically use, with rgb being by far the most common.

However, in Group Theory in a Nutshell for Physicists, Tony Zee uses "red", "green", and "yellow". This is a convention that I have not seen mentioned anywhere else, but a professor of mine claims to have learned the same notation that Zee uses.

I was hoping to learn about the origin of this discrepancy. Obviously, these are just labels, and one can call them whatever they so please. That being said, I'm still interested in what the origin of the "rgy" naming scheme is. Is it a region difference? Is it a generational difference? There seems to be almost no mention of the "rgy" coloring scheme outside of this book, which doesn't even make mention of the fact that there are other conventions, much less that they are more common.

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The oldest use I can find of red, green, yellow quark colours is Frank Close in 1979, but that seems to be a one-off since he used red, green, blue in 1975, and red, yellow, blue in 1986.

Before Zee used red, green, yellow in his 2016 "Group Theory in a Nutshell for Physicists", Kerson Huang used the same convention in his 1992 book "Quarks, Leptons & Gauge Fields", and then made the claim in his 2007 book "Fundamental Forces Of Nature: The Story Of Gauge Fields" that "the conventional (sic) now is red, yellow, and green". This certainly isn't true in the English language physics literature, but since the QCD colour names are essentially arbitrary, I wonder if there might be different conventions in other languages. I have heard that red, yellow, and green are traditionally considered the luckiest colours in China.

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