[KaijuUltraFan]

Visually it seems like the primary and secondary colors they already use (red, white, blue, yellow, green, pink, black),..just look good on film.

Brown visually wouldn't really look good on film. Neither would rainbow or clear.

And how would you market a clear toy? Clear plastic? That would look odd. Has any kid ever listed, "Brown" as their favorite color? Like Red and Blue?

I'm pretty sure the reason they use the colors they do is because they look good and track well on film and stand out from the backgrounds.