Create a stone finish for your foundation by first painting your paperclay stone foundation with a muddy grey brown acrylic paint. When this layer has dried, dry brush or dab on touches of white, black, grey and burnt sienna paints to mimic stone. Pick up a bit of paint on a fairly stiff paint brush, then bounce the brush into a piece of dry paper towel to remove most of the paint. Use the prepared paint brush to apply the tiny bits of paint remaining to the faux stone surface. In between colors, use a damp paper towel to dab at the stones to remove some of the paint and blend the colors. Do not apply thick coats of paint, just tiny dabs of color.
When your stones have achieved the effect you want, use a thin wash of burnt umber paint on a fine paintbrush to create the cement lines between the stones. Leave the stone finish to dry.
In the picture shown, the stones on the left have been finished with paints, while the stones on the right have only the base coat of muddy grey/brown for a comparison.
As a final detail, dab tiny amounts of matte acrylic finish on to the stones, and apply chalk in the colors shown to the finish before it dries. This will create the effect of lichen growing on stone. You only need a few patches, don't over do it!