So basically what you want to do from what I gather is to;
Paint the item with a wood color and seal it. Use other colors you would find in a wood then paint a semi dry brushing technique to get streaks with these colors. Seal after each application as to not destroy the work you did. the more applications the more depth and realism. Colors like orange, red, yellow, brown mixed with others to get the correct shade.
As a person who works with wood it sounds like a good way. I build guitars and use a few different woods for the bodies and necks. No two pieces of wood looks the same, even if they were cut from the same tree a few inches apart.
The key here to me is to decide what type of wood you're using. For instance if it were mahogany you would want a reddish brown color overall, although some have more orange and some more medium brown. Pine is a light white yellow that becomes honey colored with age if a finish is applied. Same for Ash and some maples which can start off white and change with age into beautiful natural colors with awesome grain. Alder is a very light brown in some cases with very very light mocha streaks. And so on.
I guess if we are talking about shovels for instance the older ones were probably northern ash which is a very hard wood. It's kind of white at first and turns into a honey color in time like maple and pine if finished. There's a softer swamp ash too. But I think the cheapest and strongest of the hardwood used in tools was northern ash, they were also very heavy woods.
I doubt they had any finish on the handles of the tools other then OD green if it were US Army stuff. Once the paint wore off I'm sure at one time they were made to paint it again. The metal parts were where you had the wear. That part would be a few shades of green to bright steel.
Crates were not painted and it looked like they used a light weight pine or whatever scraps the company that made the crates had around. It was going to be thrown away so very little care went into making ammo crates at that time. You can see the open grain of the wood because of having no finish. Anything from white pine to a slight brown, yellow or orange color is OK. But the basic color would be a yellowish, I guess you would call it light buff or sand, cream, off white, it's hard to describe the color of raw wood. So many names for color these days.
Wood left out in the rain for a while and then dried by the sun quickly becomes a gray color. So you would see some of that too. Although I guess in Vietnam for instance the sun just bleached everything. Sun can bleach things to the point of having little pigment left. So anything in the desert or Vietnam would retain it's bleached out color. If it were Europe I would think the boxes would be grayish and dirty brown from the soil.
Stocks of a rifle like the M-1 Garand were Walnut which is a dark brown. They used an oil finish and that got a nice patina over time. I would use a dark brown with a little red in there and little streaks, but very little. It would be a satin finish on it not matt or flat. Same for a pre war Tommy gun, the carbine looked like it was made from a cheaper wood as did the later Tommy guns made during the war and more of a matt finish. M14s had nice stocks too.
I am right now actually working on a diorama with some wood items and will be experimenting with that technique myself. I guess you need a lot of patience to do a little work, let it dry, clear coat, let it dry and so all for a door or gun stock.