Cannonman1,
There are many shades of the Olive color used even within the Ord. Dept. The basic pigments used were Black, Yellow and White.
(Lamp Black, Yellow Ocar, and White lead oxide.)
At different times during the War each of these color pigments maybe more available than at other times, so the actual shade of Olive can vary greatly. (from a very dark green to a light sand color) Many contractors were not required, under the contract, to paint the crates.
The lettering maybe done in different colors, often to help identify ammo calibers.
White or yellow = Standard "ball" (.58 or .69 cal. ammo of the time period. (of course on a un-painted crate, this does show up very well)
Red often = Buck & Ball, buck shot or shot shell loads.
Blue suggested a caliber somewhat smaller than the standard "ball" ammo. Like .36 up to .56 cal.
All of this can be very confusing within the crates that have survived today. Due to the fact that large amounts of ammo were placed in storage after the War and many crates were re-used and remarked.
I hope this helps.
My best,
Blair