You can start with a base coat of earth, say tan, for now. Then, go back over and paint puddles of a medium green, like a lightened olive drab. Then paint in smaller puddles with a darker green, say forest or olive green. Using a toothpick, put in some dots, using all three colors. You can put a big dot of dark green down with a blunted toothpick, and then tap in another with the tan, and so on. Then, you might want to do a real light wash with your earth color, to tie them in somewhat. The trickier part will be to shade and highlight all these tones.
The point is not to outline those spots, but to put down a big spot, then fill in with a different shade. Does this make sense?
Here's my example:
finished:
The smock in the picture you submitted would have much less earth tone than mine, but I think you could achieve the same effect. I base coated with Vallejo Air sand yellow, then laid over it with reflective green (med) and German camo bright green (light).
I hope this helps you to some degree. The first time won't be your prettiest but, as mentioned, some practice will land good results. And if you like your first attempt, you will only get happier from there.
Steve