RPA,

I don't have a wheelwright for you (although a brother of my great
grandmother ran a cooperage in SF), but I would think that you artillery item
would've been painted "field gray", the standard color for German Army Stuff
in WWI (probably WWII, also, although  Adolf may have changed the exact color
the second time around.)  I have binoculars with remnants of field gray paint
that could be matched to Benj Moore paint chips if you can't get a better
sample.  That's assuming your Krupp artillery piece was German army-- it may
have been made for export to somebody else.

Good luck,

Ralph Walter