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.

HO Scale Models

Rapido offers the 1940s and 1950s scheme.