I use Vallejo Olive Grey 888 to represent the SCC15 Olive Drab used by 1944 for most equipment like mortars, Bren ammo boxes etc.
It's pretty close to Mike Starmer's colour chips for British vehicle colours. Ammunition boxes were often made in a chocolate brown shade something like Vallejo Burnt Umber 941, but perhaps a bit more red-brown. Drop Containers scarcely survive, so I am uncertain about them, but the Olive Drab is fairly probable.