I cast scenery rocks using plaster of paris for my HO scale model railroad. I will tell you a really handy tip that produces some great results in the absolute easiest way.
Make a wash of "wet water" with black India Ink. You can find recipes for this everywhere but the general idea is a typical Windex sized empty spray bottle, 1 cap full of Dawn dishwashing liquid (or similar), and 1 teaspoon of India Ink. Adjust the ink up/down to your preference. That ink is potent stuff! A little goes a LONG way!
Spray your cobble stone street while laid flat, and the ink will settle into the mortar crevices making those areas darker. You'll get a really beautifully toned gray result because of the way the plaster readily absorbs the wash. You can spray more to get a darker effect.
Be careful not to get carried away and go too far. Your initial results will look awesome but you might want it a bit darker. Watch out because the tone can change a bit when it dries! At first the plaster is so dry it absorbs the wash and dries out almost instantly. But once the plaster gets saturated and damp, it will look darker than it really is (think of your concrete driveway after its rained and how it turns light gray in the areas as it dries up in the sun). Really damp saturated plaster castings can take 24 hours to dry all the way back to their true color.
You can't remove the ink from the plaster once it's in, so I repeat - don't spray too many coats of wash or you'll regret it.
You can also do a similar wash with cheap acrylic paints like those sold at Walmart for 50 cents per 2oz bottle.
I leave you with this tip because I've spent many hours doing MUCH more laborious multicolored painting to my rocks to get the desired tones/shadings/effect, and sometimes I shake my head and wonder if it was all worth it when the results I got in 30 seconds using this ink wash method were almost as good.
There's something about the way the plaster absorbs the ink that gives awesome relief and highlights. (The opposite result of spraying a gray paint on the plaster which would give you 1 solid color with no shading whatsoever.)