How to paint only one part?

I'm trying to replicate the Vauxhall Nova SRi from Top Gear S21E1, so how do I paint a door in blue while remaining the whole other car red?

 

unless there is a seperate option like

Select: Rear Left Door
Rear Left Door (Colorable)

If you don't see an option similar to this on whatever you are trying to paint then unfortunately you can't

OR maybe something like a paint option to select.

Beam is limited here

 

By making a livery. Kinda work but hard to accomplish compared to just repainting