As other have said you can use whatever but in real life these engines would have pulled mainly passenger cars and most would have been pullman green back in the 20s and 30s. By the time the streamline silver and brightly colored cars came along larger engines were pulling them.
The most common car it would have pulled were Harriman type passenger cars though SP had many of the more common heavyweights.
Sunset 3rd Rail made these. Note the round roof.
It appears few if any O gauge manufacturers have made green SP heavyweights, only Daylight versions but you could use car lettered for Pullman which SP did have.
This is an MTH Pullman heavyweight.
While SP did have heavyweights painted Daylight colors there were only a few dozen and most all were RPOs, Baggage, or Diners.
Again, run what you like.
edit, I should have checked out your engine first. Looks like Paper TRWs suggestion would be a good bet. Also MTH has done Railking cars in this color scheme.
Pete
