The Swift fleet (S.R.L.X.) was part of the General American Transportation Corp. (wood car division) fleet.
Although I have not researched their paint schemes for absolute accuracy, someone posted the following information to the Atlas HO Scale Forum in July 2005:
Early - Orange or yellow with a small Swift herald. Conservative lettering.
1949ish - Classic red sides with white billboard SWIFT.
1958 - Silver car with the red 'bar' with SWIFT appeared.
1959 - Silver car changed; now had the red ball with Swifts Premium on it.