How to get this green tone...?

you can just keep overlapping the layers in thin coats and the original color will start to show up from the darker shade. Adding water to the green might brighten the color too so you could try that as well but your color will always look darker on the first layer since thats what the black primer does. If you want it to be brighter then you should consider using white primer.

If most of the model, say 50% or more, is going to be Dark Angels green or any of the other darker colors in the Citadel range, then it is much better to undercoat your models with white. DA green is almost impossible to see over a black undercoat without several coats, and I mean like 5-6 coats at least.

If you only want to do part of the model in DA green, then do this: Prime the model black and then take the parts where you want to paint DA green and paint over the black primer with either white, or a much lighter green color. I prefer white myself, but I have seen people use lighter browns or lighter greens to make a darker color stand out.

Most of the elf models will be with Dark Angels Green, and some brown color on the leather parts. I have now sprayed all parts black, though, so I'll rather try using more layers on these and when I buy a new unit I rather borrow/buy a white spray box for them (it's a pain in the ass to spray the parts when their taken of those thingies they are attached to in the box, or when the model is already put together ).

You could always brush on a thin coat of white paint where you plan on painting green. It should work in a pinch and should save you several layers of green.

Cheers,

-Mike