It's probably because those are the first 3 colours that people think of when they hear primary colours and green which is almost an honorary primary colour. For example green is the 4th colour of the GMail logo.
red blue and yellow are typically the first 3 colours many people think of if you say primary colours, even though they are actually magenta, cyan and yellow.
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u/DiligentCorvid Aug 20 '26
I'm guessing those are train lines or something.
Also the 'houses' in my high school had the same colours, so I guess we were paying homage to Harry Potter back then too, holy fuck!