The color of Songbird's interior is unknown. Military AT-17s and UC-78s had a gray or brown interior, in materials similar to that used in automobiles of the period, but civil owners had a wide variety of materials and colors to choose from. Cessna did provide interior kits for those aircraft converted post-WW II, but documentation of those has not yet been found.
Instrument panels were usually black, sometimes in a "crackle" finish.
Michael McMurtrey
IPMS-USA #1746
Carrollton, TX