Crawl up underneath the gas tank area in the passenger's compartment. Wipe an area towards the back with water or a solvent to bring out the shine and clean off dirt. Unless someone did a thorough restoration and completely repainted all of the inside gas tank, the original color is probably still there. The same with the underside of the hood. Oftentimes only the exterior gets repainted. Check for the original color there, too. The inside of the firewall is also a good place that lazy "restorers" don't repaint, especially above the bottom level of the gas tank. If green can be found in any of these areas, the body color was probably originally Kewanee Green, a very common "standard" color for the '30-31 Model A.
Marshall