Load a brush with paint and drag it down the wall. Let gravity pull your hand. This is a faux technique called strie. Use adjacent vertical drags for an organized texture. Use varied strokes for a less coherent design. Drag in different directions for a non-strie pattern, but be careful. When dragging in directions other than vertical, you risk drips. Begin at the top so you can cover drips on the lower areas of the wall that may occur.
Paint directly on the wall first with one or more colors. You can use stripes of horizontal or vertical orientation, or other pattern if you desire. With the paint still wet, place the squeegee on the top of the wall. If you are substituting cardboard, place a flat edge at the top of the wall. Apply gentle pressure. Drag the squeegee (or cardboard) downward. Mix the technique with horizontal swipes, or swipes in other directions.
Place a range of acrylic colors on a large, flat palette. Array them randomly. Sweep the squeegee or cardboard through the colors. It will pick up the multi-hued paint. Place the squeegee or cardboard paint on the wall first and drag it. A diverse color pattern will result. Continue in a straight line, or alter the direction if you wish. Reload and repeat.