Hello Bill & Group!

RP CYC 35 dealing with the 1937 AAR boxcar I thought would offer a hint of help to answer your question.  None of the noted SL-SF freight box cars produced in the mid 1930s into the 1940s within their photo captions makes this clear nor is there any reference in text.  However, there is mention with respect to the ten cars converted to BX use circa 1942.  These received black underframes and the trucks, like the sides, were done in Pullman green.  Perhaps this generally suggests that underframes were black and trucks were done same as sides but, perhaps, it more properly reflects passenger car practice.

HOWEVER, in RP CYC 24 is the story of USRA DS boxcar convertions to steel sheathing.  The Frisco was a big participant with this program but none of the photo captions make clear the painting practices.  Same with the text.  BUT, this reference reminded me that Sunshine Models produced an early kit, No. 4.2, of the SL-SF rebuilt boxcar.  Within the instructions our Saint Martin refers to a Frisco paint diagram that -- 

"indicates that the entire exterior carbody, including underbody, was painted freight car red.  Trucks however, were generally black."

Perhaps other references might offer more evidence, Bill, but this seems like a reliable suggestion for cars repainted by the Railroad.  It seems entirely possible that new car painting sometimes reflected the builder's preference.  If your model is of a new car, you ought to seek out that builder's documentation.

Regards from 27 degree western Penna.  Mike Schleigh in Grove City