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Mix up two vats of powdered joint compound and paint, one a shade lighter than you'd like and one a shade darker. The mixtures should have the consistency of creamy mashed potatoes.
Load a masonry trowel with the lighter color compound using a plastic spatula. Press the trowel down and work the compound into the wall. Use a lot of pressure if you'd like a smooth look. Let the trowel skip over the wall a bit in places where you'd like a streak of "stucco." Whenever you stop and start, blend the new compound into the edges of what's already on the wall.
Tip: If you and a friend are working together, don't start in opposite corners of the room and meet in the middle, or the stucco patterns won't look anything alike. Work where you can both see the pattern you're creating.
At this point, you can also make other textures with tools you can buy. For example, drag a texturing squeegee over the compound to make a ridged pattern, or you can use a window squeegee and cut notches in it yourself. Or, use a piece of corrugated cardboard or even the bristles of a broom to make other patterns. If you don't like the patterns you've made, smooth them over with the trowel immediately and start over.
Trowel the second, darker color on to the wall. Apply globs of it here and there on top of the lighter color, then smooth it over with the trowel, working on small areas at a time. This will give you subtle shades of the same color. You may want to texture some more at this point, too. If you don't like the ridges once they're dry, knock them off with a putty knife or sand them down.
Wait a full 30 minutes for the compound to dry, and then seal it with water-based polyurethane sealer.