I'm not totally sure about the MD-90's, but I read that somewhere I think that Delta was keeping them in the old color scheme. For what reasons I don't know. If you go to the Delta Airlines website, you will see that the MD-90 under the fleet section is painted in the old colors along with the L-1011, while all the other planes in the fleet are in the new c/s. Again, I can't provide an accurate, but mabye this will help.
"Bangkok Tower, United 890 Heavy. Bangkok Tower, United 890 Heavy.....Okay, fine, we'll just turn 190 and Visual Our Way