This USA Today article has some great photos of vintage Amtrak interiors. The last photo shows the standard Phase III interior for the Amfleet cars.
Out of the Amtrak archives, here is an Amfleet Food Service car interior.
When I was riding Amtrak in the late 80's these colors were pretty consistent across the fleet with some minor variations here and there.
I have the same set of Phase III Lionel cars and need to do some interior improvements to them as well someday. I will probably paint to match my GGD Amfleet cars.