An even darker green should do. Just add black to your base green colour and go for it!
I use water-based (not acrylics!) artist oils for my washes, mixed up with a drop of washing-up liquid and some white vinegar, plus the water obviously.
On enamel, you can use acrylic and water-based paints without any risks. If you chose to use enamels, make sure your paint is really very dry (several days!) and maybe give your model a coat of acrylic varnish before the wash, just to be safe!