Making things paintable is not that difficult as it seems. You can use the standard way, or create your implementation.
The standard material (Factory_Inst) used by SF and provided by the SF modding unreal starter project is the best way to implement the ability to paint.
It gets implemented by the
FGBuildable-class und changes the primary
and secondary material parameters of the Factory_Inst material. You can
find it in the unreal starter project under
This material implements also multiple other surface materials. It works as an atlas map.
This is a simplified representative atlas map for the material you can use to UV map your models.