When it comes to painting stone i always look at what the setting is! So the age of the building, its condition and what has been happening there as all these things should really effect how you paint the piece. The basics for a medieval type building will start the same with a black/dark grey undercoat then a few drybrush layers with your colours getting lighter.
With the first couple of drybrush layers i'd make these quite heavy but not so heavy and piant overloaded that you see the brush strokes (it has to look natural). When i paint stone i would hit it from all directions and also gently drybrush in circular motions, even dabbing at it in points. This way you make make a flat surface look like it has alot of texture to the eye. After a bit of drybrushing and depending on how you want it to look i would use a few washes. I use alot of greens and browns in specific areas picking out parts that would collect more dirt or where water would flow etc After this i would again drybush with my lightest colours and really gently drybrush in circular motions again and pick out the finest points. If the model was a diplay piece i'd then take the next step and use weathering powders to really add to the realisim and give it more earthly tones.
Here is a link to a tutorial i put on here back in 2008 - Flag stone bases
. It is an old thread but it may help you see what i have just been talking about.
Hope this helps and good luck with the project.