Copy a wall you admire when designing your faux paint version. The easiest stone wall takes only two colors of paint: a base coat and a darker glaze coat.
First cover the wall with the base coat. When it dries, mark off the stones with thin painter's tape. Painter's tape is blue and peels off without leaving a residue or removing the underlying paint. Sponge on the darker color glaze, mimicking the shading of real stone -- some stones are darker around the edges and lighter in the center, and some are just the opposite. When the glaze is dry and you remove the tape, soften the edges of the stone where they meet the "grout" lines running between them by painting lightly around each stone with a small artist's brush and thinned glaze.
Avoid perfection to make your faux stone look completely natural. Paint a stone wall covered partly by plaster, as if the stones had been revealed when the wall aged and the plaster fell away. Paint stones with a base coat and shape them with painter's tape but make the stones very irregular, as if they are "found" stones, not quarried and cut to match. Shades of brown look real and a little bit dirty: very convincing. Make them pop with a black edge, brushed or sponged along grout lines, using a ripped cardboard guide. Sponge the merest hint of white on the tops of stones, where they are likely to be hit by sunlight.
Cover a wall in a bathroom or hallway with faux stone wallpaper. It's much faster than painting the wall, and there are a number of styles and colors available to suit your décor. But don't let the faux stone wallpaper just end at the ceiling -- it will look fake. Paste a matching or complementary border along the ceiling edge. Use stone arches or a keystone border or another design to top off the wall with all the polish and definition of a real stone wall.
When you want the look and feel of stone but can't manage the price and cost of labor, use stone veneer. Thin panels of real stone adhere to the walls and look as if they have the depth and solidity of whole stones. Install the veneer as panels or as individual pieces, which allows for customizing but does take more time. You don't have to worry about weight or a major construction project with veneer. There are also stone veneer wallpapers that come in big tiles or panels for gluing on the wall. The surface is real stone, very thin, but it's backed by adhesive paper for easy installation.