Tape around the wall behind the sink. Place a plastic sheet over the sink and counter.
Add a few drops of black and raw sienna tints to the primer. Stir until the tints are completely incorporated. Adjust the amount of tint until the color looks like grout. Use more brown for a warmer grout color.
Roll the tinted primer on the wall. Spread the primer out to leave as little texture as possible. Fill in any areas you missed with a foam brush. Follow the directions on the primer's label to see how long you must wait until applying the paint.
Use a tape measure and a piece of chalk to mark out an even grid for your tile design. Place a strip of narrow tape along your grid to create faux grout lines underneath the tape.
Roll on a smooth, even layer of paint to create the tiles. Use a foam brush, wherever necessary. Add a second coat of paint if the first coat looks uneven. Allow the paint to dry.
Use stencils if you wish to create decorative faux tiles. Apply a small amount of acrylic paint with a stencil brush. Tap the brush lightly and evenly on the wall to distribute a thin layer of paint.
Mix a small amount of tint or acrylic paint into some glazing medium to create a transparent version of your tile color. Brush the glaze onto each individual "tile." Blot the glaze with a crumpled plastic bag. Soften the mottled effect with cheesecloth, if desired.
Carefully remove the narrow tape to reveal your faux grout. Touch up any areas of bleeding paint with your tinted primer, if necessary. Remove the outer strip of tape.