Some say graffiti tagging, some say custom paint jobs.
I saw a Chicago Great Western covered hopper in 1997. It was absorbed by the CNW in 1968.
I have no problem at all “mixing’em up” on my '70’s layout, when Chessie took over the B&O, C&O and others. I have B&O, C&O, ACL and other liveries in the “family”, as I do recall the old paint still on those cars, along with Chessie paint during that time. I even still have Erie and DL&W cars, along with EL cars as another example, among many instances of these post-merger mixes. As Mike Kieran stated, the acquiring road takes on the acquired road’s “brand”, if you will, so repainting the rolling stock is not a big priority.