Use a level and a pencil to draw vertical lines down the length of the walls. Place the lines about six inches apart. Draw horizontal lines down the wall about 12 inches apart. This will make a basic 12-by-6 size for the diamonds.
Place a dot in the middle of the top, bottom, right and left sides of each rectangle. Connect the dots using a ruler or level to make a diamond shape inside each box.
Cover every second diagonal row of diamonds with green painter's tape.
Paint the outside edges of the diamond with a small paint brush. Allow the edges to dry. Fill in the rest of the diamond with a larger paintbrush. Allow the diamonds to dry for two hours.
Remove the painter's tape. Paint the remaining diamonds with the second color by painting the outside edges first and then the interior of the diamond shape. This prevents color bleeding. The resulting pattern will reveal a harlequin pattern with alternating colors of diamonds.