Could these be used as Ranger colors at some point, even if 2 of them aren't colors?

Brown, Rainbow, and Clear (invisible) but you can see some of the suit like the visor, collar, gloves, boots, and team logo.

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.

Has there been Brown Kamen Riders? Also yes clear plastic.

Kamen rider is different due to it's armored designs, rather than skin tight suits.
And yes, there have been brown riders (Gridon is the first to come to mind, but I know there are others).

Impaler from Ryuki came first as a brown rider followed by Eiki and Habataki from Hibiki and Punch Hopper from Kabuto. Yes Punch Hopper is brown. A dark and a tad desaturated brown.

I was unsure whether to call Punch Hopper brown or copper. I remembered the Hibiki riders, but I couldn't recall their names, so I just didn't mention them.
Straight up forgot about Imperer, though :lol

A copper and/or bronze ranger would be cool.

Yeah, I thought Punch Hopper was dark grey at first long time ago but yep, if the figuarts and his zector are any indication his armor is of a brown hue.

It happens. :lolol

Ditto. We already have enough gold and silver rangers I think personally so a bronze ranger would be nice for a change of pace.

Kamen Rider Ketaros (had to look up the name, I always get those three confused).

What colors would you guys like to see used that haven't been used before?