You could always trademark colours. I know in the early naughties yellow pages had trademarked their yellow #ffdc00

This has come from Cadbury's battle with Darrel Lea. Both chocolate makers have used the colour purple for decades.

Last I heard, Cadbury lost and Darrel Lea could use purple for their chocolate still. However, Darrel Lea have gone to a brown paper theme on their chocolates. So, I don't know what's going on. Maybe Cadbury bribed Darrel Lea to change their branding after court action failed? HEaven knows Darrel Lea could have used a financial boost this past decade.

Kinda, usually if it's a big part of your brand. And it only applies to that companies area of expertise. Like they couldn't stop a car being made with that color. But you can't make chocolate eggs like that. You can't sell tractors the exact same color John Deere makes them.

It's still bullshit though.

Not sure it's enforceable in every country/jurisdiction, but yeah, someplace they can. There was a big controversy about it about 2 years back.

https://thehustle.co/can-a-corporation-trademark-a-color/

First link I found, there are properly better sources out there, but that'll do for a general overview.