Purchase ivory, dark gray and brick-colored paint.
Mix the following colors in separate containers: Ivory and dark gray equals medium gray,ivory and brick equals light brick, dark gray and brick equals dark brick. The exact shades are unimportant so long as you are happy with them.
Lay down a tarp to protect against drips and mask off any windows, doorways or trim with painter's tape.
Paint the wall with medium gray paint and allow it to dry.
Dab on dark gray paint with a sea sponge to create a mottled texture. Allow the second coat of paint to dry.
Lightly dab on light gray paint for highlights. Let the entire wall dry overnight.
Tear strips of 1-inch painter's tape down the middle. Stick them back together to form 1/2-inch strips with torn edges on both sides.
Use a level and yardstick to draw a brick pattern on the wall with a yardstick. Use a pencil and keep your lines as light as possible.
Gently apply tape over the lines to mask off the areas where the mortar will go. Don't press the tape down hard or it will pull off the gray paint when you remove it.
Paint a coat of the brick color over the wall and tape. Dab the wet paint with a rough green sponge for texture and allow the paint to dry.
Sponge a few random bricks with the dark brick color and a few with the light brick color. Bricks in real walls are rarely all the same color.
Drybrush all the bricks with the dark and light brick colors to give them more texture and detail. To dry brush, dip the brush in paint then wipe most of it off on a newspaper before brushing the wall. It will create subtle streaks and lines.
Allow all of the paint to dry overnight.
Peel off the masking tape to reveal the mortar.
Paint dark gray shadows on the mortar under each brick if desired.