I`ve found that, with the large amount of hydrostone plaster buildings I`ve built over the years, painting them can be a bit laborious. True, you can look at it as a labor of love, bringing out every detail of a carefully built piece. Or you can look at it as one more step before the building gets on your table where it belongs.
After hourse of casting and more hours gluing, I was looking for a way to both make my buildings look good, while still avoiding the painstaking process of brushing 3 layers of color over every building I made. Which is a lot.
What I came up with was to use Woodland Scenic landscaping colors in a diluted form.
It made sense really, as Woodland Scenic has built a commecial empire on realistic depiction of outdoor terrain. Who better to look for materials to colour rock with...
With that in mind, I made a big mix of color, stone grey is good for a Northern look, mixed at about a 1 to 10 ratio, but I`m not exact.
Then I just dip the completed slab in, and pull it out.
Done.
For shadowing effect, I use a wash of water with a bit of black paint (craft acrylic works fine) and a bit of Future floor polish. The Future creates a good wash and adds a protective layer to the model.
I`m going to play around with different stone colors on top of the grey, applied with a sponge for variation, but I havent done that yet.
I`ll let you know how that turns out.

 
For me, I got started with building terrain in a trial by fire. 8000 HirstArt bricks for a class of Grade 5 kids. I`d only planned on doing 2000 (still an awful lot) but they proved so popular that every kid in the class wanted one.
Since everyone now had a nice castle, they needed some nice scenery to with it...
So here I am 3 years later with a lot of terrain under my belt.
A few things I`ve found.
1) bricks are slow to build and make.
I lent out my Hirst molds to a friend for his kids and now we dont talk. I dont think he had any idea of the time it takes to make the bricks, let alone glue them together. They are super versatile and fun to build with, just dont expect to build the Great Wall of China in a weekend.
2) terrain building is the ultimate recycling.
Well, maybe not the ultimate, but styrofoam tends to last an awful long time in a landfill if it gets there. But it can last just as long on your tabletop if you carve it into a nifty piece of terrain. The same goes for old pieces of electronics and plumbing. But dont just take my word for it. That advice comes straight from the guys at games Workshop.
3) reality isnt flat
Surprisingly enough, that one took me a while to figure out, as I kept making terrain that just didnt look quite right. I had my grass flock and my foam bushes, but the scenery was flat and boring.  Thats where textures come in.
To get the best look for your terrain, use a variety of different textures and colours on the ground floor. Be random, but with a plan, like nature is. The only drawback to this is that it costs a lot to get set up with all the materials you need. Because these, unlike foamboard, dont get recycled.