Combing is a technique for applying texture to a wall using a rubber or cardboard comb. Combs are available at most hardware stores in pre-made patterns or can be made at home by cutting notches into a rigid cardboard edge. Combing is accomplished by painting a surface a solid color then applying a glaze over the top. While the glaze is still wet, drag the comb through the glaze to create lines in the top layer. This will dry and create a combed wall surface.
Specialized custom glazes are available at most hardware stores. But glazes are easy to make yourself. All the glaze really consists of is paint that has been thinned to the point that it is translucent. Thinner paint allows some of the base color to show through.
Typically you will want to select colors that either complement or contrast one another. Keep in mind that the greater the difference between the two colors, the bolder the wall will be. For a more subtle look, try combing with two different shades or tones of the same color. If you want the glazing layer to be more prominent, use the darker of the two colors for the top coat. For a more subtle textured effect, use a light color for the glazing.
There are many styles of combs that offer a variety of lines. Another way to vary the final effect is to experiment with the direction you drag the comb. Dragging a comb in straight lines will produce a somewhat organic yet consistent pattern. You can also try dragging the comb in wavy lines or spirals. You can add a more chaotic touch by dragging the comb across the same area in several directions. This will allow you to crisscross or crosshatch your marks.