For speed (and as an experiment) I tried DM Scotty's sponge method, and I think it worked well. A few people asked for details, so read on.
You will need; paints, I use cheap acrylic craft paints, they are quite good enough for this. There are many brands, I just use what I can find. For this colour scheme you will need dark and light grey, dark or mossy green and a reddish brown. A very light grey (or white to mix in) is good for highlights, but by this stage using model paints isn't too expensive. You will also need a sponge. I use triangular make up sponges that I buy in a bag for £1 from a discount shop. Anything with a medium texture will do; if you've got the sponge from a pack of figures that's about right, just cut it into a useful shape (triangular is good). The triangular shape helps when adding detail and highlighting. An old plate or takeaway container to hold the paint, and finally gloves, as this is a very messy process.
Firstly build, wash and black undercoat your Deadzone terrain piece.
Then give the whole building a fairly heavy sponge coat of dark grey. I use a very dark shade, almost black, then gradually add light grey so the effect is mottled. If you have a triangular sponge (and its easy to cut one to shape) use the largest side for quickest coverage. The action is a dabbing motion rather than painting it on, and moistening the sponge first (though not soaking it) helps. Dab the sponge onto some old newspaper or similar to take off the excess and get a good amount held within the sponge. Be careful when applying; too heavy handed and you will end up with a smear rather than the subtle texture you want (and yes, you can see that several times on this). If that does happen, repeated dabbing will diffuse the patch of colour somewhat and you can always go over with a darker colour again. You can start to pay attention to where you want shade and highlights, but most of this comes in later stages.
Rinse out the sponge and then add some different colours. This gives a much more naturalistic result, as buildings are rarely one shade, even when made from just one material. I sponged on patches of green and brown here, though I realised that I used a sandy brown rather than the reddish brown I prefer (wonder where that paint has got to?). Looking at it now, I think this would give a good effect for any GCPS military vehicles, hmmm.
Don't worry about exactly where this goes, just aim for a fairly even coverage. It's going to be mostly covered up in the next stage anyway.
You can just see the sponge I use to the right of the building.
Now sponge over again in a lighter grey. This time pay attention to the areas you think would be highlighted. You will be covering up some of those patches of green and brown, but if your sponging is light, some of the colour will show through. If you have a triangular sponge, now is the time to use a smaller side to give better control, especially on those edges.
Finally for now, highlight the edges and anything else you feel needs it with a near white (or very pale yellow or brown) I used a craft paint that is very similar to Bone White for this, as I find pure white a bit stark. I have tried to emphasise the tiled effect on the roof by catching the edges with the highlight. Use the edge of the sponge to catch these highlights. With practice you have a good amount of control here, so quite delicate effects can be achieved (and yes, I did rush this for the blog)
That gives a useable result, but a bit of detailing will really make the building pop.
Next time I'll show you how.