The trick to getting a nice stone effect is watery, blotchy layers. You're going to need a coarse paint brush, a small dish, water (I use a small water bottle as I'm less likely to knock it over and destroy all of my work), and some acrylic paint in stoney colours.
I used Revell Aqua Color in sand, dark earth, earth brown, and medium
grey - the light blue at the top is what I used for the woodwork - and
Liquitex burnt umber.
Drip a little bit of your lightest colour paint in your glass dish, and add enough water to make a nice coloured water. Splortch patches of colour on your wall, leaving white areas and let it dry thoroughly.
Move on to the next colour and do the same. Overlap some of the previous colour as well as covering more of the white plaster. Allow each layer to dry completely or you'll end up with a muddy look.
This is after the third colour. Now all the white plaster is covered and I've started adding a bit of a weathered pattern - drips from the eaves and a stain where the stonework is cracked.
The final colour before adding the grout is the grey which I applied fairly uniformly over the entire building.
For your grout, you can either go light or dark. I prefer light as that's how our local buildings look. Again, you want your paint to be watery, but it can be a bit thicker than the stone coats.
Since the grout area is actually exposed wood, it's going to suck up a lot of paint, so you might find you'll need to go over some areas more than once. This is the tedious part.
After the grout dried, I used the bunt umber to further weather and stain the building.
Once things have dried, it's time to seal the paint. I like using Mod Podge as it protects and smooths the surface - the putty can have some sharp edges and it loves to eat cat hair. Podging it helps all of these issues.
The only downside is that even the matte finish gives the stone a bit of a shine. In the image on the right you can see the difference between a Podged wall at the bottom, and the un Podged strip at the top.
While the outside dried, I finished up some bits on the inside such as the grout in the side room and installing the door frames.
The walls and ceiling got a fresh coat of white paint and some water damage.
The Magpie's Nest is just waiting for me to finish the second floor and to get started with filling in the window and door frames.