Does the museum have any later pics of their fleet? Many fleets repainted the area between the side sash in black in later years. If you could find a pic of the fleet circa mid/late '70s > '90s you could duplicate that and still be "correct."
Unless, of course, they want it "as delivered new."
If you look at my avatar here, and then look at my avatar over on BNO, you'll see the same bus with different paint.
Personally, I really like the '04 and '06 with the black window treatment. . .
FWIW & HTH. . .