This idea probably only works for me, but here goes:

Because of my synaesthesia I see the numbers two three and five as blue green and red respectively. Red blue and green are often used as the primary colours of light (although they're not the only options) and two three and five feel like the primary colours of numbers to me, because they're the three lowest primes, so you can mix (multiply) them together to produce most other natural numbers.

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