Wakefields book has good info (The Fighting Grasshoppers) on page 151.
The Erection and Maintenance Instructions for the L-4s just says "shade 41, olive-drab" AAf spec no. 14106 on the upper and side surfaces. "shade 43, neutral grey" AAF spec no. 14106 on lower surfaces.
Take a trip over to OKC and the artillery museum and you can see one exactly as it was when they took it out of the crate.