The US army green color from when they wore monochrome color BDU's from WW2-Vietnam is called "Olive Drab" or
OD.
I've not used it, but from the online swatches, Death World Forest
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat800002a&prodId=prod1500028a
looks to be the closest Citadel Base shade.
Really though, if you want dead-on
OD, consider just buying the appropriate color from a military specific paint line.
Vallejo has Heavy Green (146) or Luftwaffe Ca. Green (823)
Model Master has a standard "Flat
OD"
http://www.testors.com/products/573482
as well as other variations.