Depends what you have Steve!
Ultramarine is a good base for mixing as it tends to dry nice and matt, try adding Yellow Ochre or something similar to make the green. If you want to make this a dirtied shade, as the dyed cloth was known to discolour easily, add in a little touch of Cad Red or for a darker, dirtier effect some Burnt Umber. Shade with black, highlight (keep these subtle) with a mix of white and Yellow Ochre.
Try that and see how you get on.
Einion