The caboose in the picture is 05635 that was home-built a number of years ago from stock car 5635. It is NOT 0503 which today remains behind the engine house under the temporary canvas enclosure a the very end of one of the RIP tracks. So the color does not HAVE to be historic and can match the passenger cars I suppose, although to be historic maybe it should BLACK.
Prior to this year's rebuild, 05635 did not have outside sheathing and looked like, well, an outside braced stockcar Incidentally, 05635 was featured in the opening railroad scenes in the movie "Indiana Jones and the Last Crusade".
05635 had to have a lot of the underframe replaced due to advanced rot and decay. The fact that sheathing had never been applied to the outside of 5635 may have contributed to the side sills rotting out. On a normal stock car the "open sides" allow moisture to dry out. However, when the framing is closed in on the insdie of the posts and braces, rot does its dirty work where it can't dry out quickly. A lot of the old rider boxes are suffering the same fate because water/snow gets down inside through the window openings and stays there on the side sills where it causes rot.
Here are a few pictures of 05635 (as well as 0503) in 2004 when the rebuild(s) began: