In years past most railroads had a common paint scheme for their line side and company owned structures. For the late Espee it was a light yellow with a kind of rust brown trim and a green roof. There are many structure kits from Atlas Walthers and others that are excellent structures but are in different colours. By painting the "Company Owned" structures on your layout in the correct paint scheme for your road the layout will look tied together. One example is the Atlas passenger depot. It is a common structure on many layouts. The depot comes in red with a white trim and dark grey roof. By painting it and adding some details it can be an enhancement to your layout as well as getting the plastic look gone. The attached photos are of the company owned structures on the layout.
This is a very valid point and thanks to Mr. SP for the illustrations. More discussion and comments on this issue may be found in the Inspection Pit Forum.