In all my years seeing it and riding behind it, it has always been the pale yellow. It was last painted after the roof collapse, as it was on the main access track in the roundhouse when the roof came down. Since then, it has been on display next to the car shop under the cover seen in the below photo, contributing to the decent paint job all of these years later. According to museum volunteers, it is nearly 100% original mechanically and electrically and they opted to park it, only running it enough to keep it operable. Basically, a preservation move although they would have no problem firing it up once batteries, water, and fuel were put back in.
MTH did the PM 11 in it's Chessie 8401 scheme, which was the scheme and number it wore on the C&O/Chessie. I've got one and has been a great little engine. Below is a photo I took last Christmas of the MTH model with the real one.