Right! So this is nearing completion. I wanted to add some variation in the color, while still maintaining the overall grey stone. The effect is actually a little more subtle, but the flash seems to have reacted strangely with the oil paint. I'll some properly lit photos once everything is finished.
I used oil paints and thinner to add a bit of grunge to the board. It's basically the same technique military modelers use to weather vehicles. You apply small random dots of oil paint to your model and then use a brush loaded with thinner to create a wash. Then use paper towels to soak up the excess and any bits that might pool up. The random effects give a very realistic finish. This is super easy. You really can't mess this up. If you think there is too much color, just keeping going over it with thinner. You can even work back into areas several hours later.
Here you see a good before and after. The painting is done (minus painting the edges a neutral grey) and the final step will be adding snow. I'm going to add small drifts in corners and on ledges and add scattered snow flock. I don't want to cover the board so this will be done sparingly and in several layers so I can build up the effect gradually. I am also going to scatter some icicles around the walls as well.
It's getting exciting now!