[ShiShiLeo]

A lot of old super heroes and cartoon character colors were dictated on random stuff like the cost of the ink for the comic or colors on the action figure.

For example, the original Gundam was mostly all white and it was changed to Red, Yellow, and Blue both because those colors would make it more interesting to children, and because those were common colors used in toy manufacturing.

Bright nonsensical colors were synonymous with comic book heroes because it helped them stand out on the page. I don't think there's too much to read into it here.

Like why was Nin-nin/Comic purple and yellow? It's just part of the design language of Kamen Rider.