I haven't found much. GMC trucks are optioned differently, but a GMC brochure for '55 shows the following interior colors:
Harmony Gray and Panama Cream,
Aqua Blue and Dover White,
Seminole Brown and Panama Cream,
or Delta Green and Aspen Green
The same brochure also says of the upper dash "The textured paint is gray on standard models and blue, green, or brown to match interior trim on Deluxe models."
http://www.oldcarbrochures.com/stati...20Cabs-03.html
The GM Heritage Center restoration packet for '55 Chevrolet trucks indicates that floor mats are available in 5 different colors.
Too bad it's so tough to find a 55 2nd series or '56 Truck Data Book.