hi john
most of the guys i know take a chip of the original paint if some can be found), and have it analysed at a paint distributors. from there they work out a formula to give the shade you require. having said that, someone on this forum will likely have said formula written down and can share it with you. there are a gazillion shades of olive drab...just line up a dozen gerry cans and you'll see what i mean.
mike