I'll offer my thoughts on both your questions as the should relate to any type of model, brass or plastic.
Builder versus Railroad/Owner: There are, and at one time a number of Builders (manufacturers) of real Locomotives and they would produce Demonstrator locos, usually painted in an advertising scheme of their own, for the use and evaluation of Railroads who showed an interest in buying new equipment. Railroad/Owner would, should apply to the individual Railroad paint schemes applied to the model, but would also include any of their specific modifications, or varieties of included ancillary equipment fitted to the locos, of their choosing, known in the modelling world as RRoad specific detail inclusions. There is also a sub species of locos owned by leasing companies who don't have an actual Railroad. They too will normally have their own paint scheme, or sometimes have retained the paint scheme of the Railroad they came from if purchased, used. They usually have the original RRoad's name painted out and replaced with the lessor's name or mostly their "Reporting Mark", a series of letters assigned to them to identify that owner.
Painted v Unpainted value. This could be a little harder to judge value of. When model makers offer unpainted versions they usually come with additional detail parts to finish the model in a Railroad specific type of that loco, of the buyers choice. Painted ones in a specific Railroad scheme will have had it's details installed or sometimes supplied, also painted, to be applied by the modeller. If an unpainted model, they usually sell new, at the same price as one already painted. If model manufacturer painted, the scheme it is painted in can substantially affect it's resellable price, particularly depending on the demand or the sought afterness of the loco model. This is also affected by how many might be "in the market" for it at any given time. So..pot luck.
