Hi guys,
I knew that I could count on you when it comes to advice. That's why I wanted to move my subject from the painting section here. You are all so helpful:)
Before any replies appeared I've already finished my test piece, you can see it below:

It's not very complicated when it comes to painting. A black base color, drybrushed using various shades of gray and light beige. After that a light drubrush of brown on the edges of the rocks and some dark green to add some weathering and breake the gray color. I'm still trying out different schemes, so if you have any advice or ideas on how to improve this feel free to post:)
@Grumpy: suprisingly our techniques are pretty similar. I'm thinking about using different shades of gray for the first drybrush layer, to make the stones color varied.
@BigJon: this is going to be a medieval tower wall. I still have to add different shades of gray to make the "mish-mash" you are talking about. Also some dark green washes might come in handy.
@Sticky: that's a cool technique you're talking about. I will have to give it a try. The wall that's visible in my original post is made from two wall panels, that were built using some home made stones. If you want more info, you can check out the scratchbuilders section.
@Scott: yup, I've also thought about using some tan to change the gray color a bit. I will also use some washes, to make the wall more realistic.
Thanks for all the advice, I really appreciate it!
cheers,
dsc.
PS. the picture might be a bit too light, in reality the wall is not so "white'ish".