You may want to check several different places on the jeep to determine the "original" color. Through the years, jeep owners had a tendency to swap in parts from other body styles (2A, 3A, M38, etc) so you may end up with a "factory" red windshield, a "factory" green tailgate and a "factory" OD green fender.
To my knowledge, there is no way the factory color was identified by tags on the jeep itself. (Later model vehicles usually have RPO codes on a tag or sticker somewhere that would identify the original color).
All part of the fun!