When I apply a uv map to part of a mesh (eg. putting a red number on a black car door) the rest of the mesh remains white in the render. Is there a way to do this which does not involve having to uv map the whole door.
You can make part of the uv-map transparent or you can decal the number on using an object mapping with an empty. Since you already have the door uv-mapped search for “transparent and uvmap” – this question gets asked all the time.