You can do it using Vellejo Very Dark Green, a bit of Khaki and a touch
of white. Start by getting the green part of the equation done. I want
to say that the green/khaki part of the mix is around 4:1, but milage
varries, so just use that as a starting point.You're looking for an
approximation to the color 179 French Artillery Green from Humbrol's
line.
Next, add a drop or two of Black, depending on how much paint
you've mixed up. You'll want a dark O.D. Green. Then start adding
white, a drop at a time, until the appropriate tone of Gray is
achieved. That should get you there.