Paint your surface with the lighter of your two colors. Allow the surface to dry completely before going on to the next step.
Divide your design surface into a grid of rectangular shapes. Each rectangle should be twice as tall as it is wide. You can use 1 by 2, 2 by 4 or 3 by 6 rectangles, for instance. Draw lines over your painted surface to indicate the rectangles in the grid.
Find the center point at the top, bottom and sides of each rectangle. Draw a pencil line from the top point to each of the two side points, and from the bottom point to the two side points. This will create a diamond shape. Draw diamonds in the same manner on all of the rectangles.
Place painter's tape outside all of the lines that create the first diamond shape. Skip one rectangle in a row, then tape off the diamond in the third rectangle. Continue taping diamonds, skipping every other rectangle, until your surface is covered.
Paint the interior of the diamonds with the darker color. Cover the surface completely, but do not allow any paint to bleed beyond the tape. Allow the paint to dry completely.
Tape off the diamonds in the rectangles that you skipped, then paint the interior of these diamonds with the darker color. Allow the paint to dry thoroughly.
Remove all the strips of paint to reveal the harlequin diamond pattern.