The Paintbrush tool paints block patches on your quilt using fabrics or colors from the Sketchbook. You can also use the Paintbrush tool to color your borders, cornerstones and sashing. For a fast method of coloring borders, use CTRL+click (Command+click) on one side of a border to make all four edges color at once.
You can also Drag and Drop a fabric swatch on a patch in the layout. Select a fabric in the palette. Click, hold and drag that fabric swatch from the palette to the quilt layout. With the arrow cursor directly over the patch you want to color, release the mouse to drop the fabric on that patch. Hold the CTRL (Command) key down to set the fabric in all positions, or the Alt (Option) key to set in alternating positions when released.