Painting realistic looking stone walls for jump fillers is relatively easy. It does require some time and patience because it takes a few steps and requires time to let the paint dry.
Decide the color you wish your "cement" between the stones to be and use this as your base color. I wanted white here.
To draw the stones approximately the same size, I used a large sponge and then did free form stone tracings around the sponge.
I chose gray for the stones. You can also choose brown if you like. You need to have a medium gray paint, a dark gray paint and white paint. For smaller projects, a quart of each should be enough.
Paint the stones that you've outlined in the medium gray paint.
Next you need to mix half medium gray and half white paint in a disposable bucket to make light gray paint.
Take some plastic cups and pour the medium gray, dark gray and light gray paints into individual cups.
With a 1" or 2" flat brush, quickly apply the light gray paint across the top and the center of each stone.
With a second brush, quickly apply the dark gray paint to the bottom and sides of each stone, also adding a few dabs in the middle of the stone over the light gray paint.
Only work on two or three stones at a time during this process.
Place a small tack cleaning sponge inside a plastic shopping bag. Using a quick motion, dab the plastic bag over the fresh paint, blending the light and dark paints. The more rapidly you do this step, the more realistic the stone. Don't try to blend or swirl the paint, but quickly pat the surface in rapid succession. Don't worry about going outside the lines that you've drawn for the stones, you will touch that up later with white paint. Wear disposable gloves during this process, as your fingers will become covered in paint.
After the paint has dried, go over the edges of the stones with white paint so there is a clear line between the stones and the cement. The darker paint may leave a bit of an "image" under the white paint, but it gives a realistic effect of stone underneath the mortar.