First off, the VGN and C&O gons had offset sides, while in true Pennsy fashion the G23 had ribbed sides. The car was built in 1919 and lasted until 1952, so for that era Freight Car Color would have been applied (I have no reference for this, just how all other classes of freight cars were painted).
There is an article on the G23 in The Keystone Volume 49, number 3 (Autumn, 2016). No mention of color is made in the article, but it does say that by September 24, 1919 (5 months after building), the car was stenciled to be used in captive service between Derry, PA to the coal dumper in Canton, MD.
There are photos of the car in this old OGR thread:
https://ogrforum.com/...-battleship-gondolas
You can see the tiny "captive service" lettering above the road name in the photo with load. Note that the load in that photo was a test of the car when overloaded to 116.4 tons (vs. capacity of 110 tons).
To be accurate, the car would need to have ribbed sides and (for all but 5 months of it's 33 years in service) have the captive service lettering above the road name.