McLaren Sabre has unpaintable Carbon Fibre parts

Red tinted: paintable
Grey: unpaintable

Both surfaces were originally Carbon Fibre

I wouldn’t get your hopes up. Similar situation with the Valkyrie AMG Pro.

It’s not though

That is painting bits onto the car

This is changing bits of the car, plus, the painted bits were only available on one livery on that car, while the carbon fibre bits here are part of the car, on which you can already paint other carbon fibre bits