Colour shades are what can be seen on old houses around here, which are built from grey granite (which can have slightly different shades in grey-blue-brown, a few are a bit pink), and some very thick slates to fill the gaps, and probably a few other stones.
I used balsa with some layers of "enduit de rebouchage acrylique" ("acrylic spackle" is it?) and when it was dry I quickly flattened it with a large file (a file for metal, not for wood). Then I carved it (but not deeply) with a sharp pencil, it goes fast this acrylic stuff is soft enough.
I was thinking about a darker wash to fill the carvings between the stones, but after more thought I won't do it, because traditionally the stuff that was put between the stones (before modern cement) is a mixture of lime and sand, its true colour is a very light yellow-grey, lighter than the stones.
Well I still don't know how all this will look when finished, we shall see...