To make swirled macarons (rather than ones dyed a single color), use a long wooden skewer to draw a line (using gel food coloring) from the bottom of the piping bag (where the tip begins) to the top of the bag. Use red gel food coloring on opposite sides (ex: 12 o'clock and 6 o'clock) and blue in between the red lines (ex: 3 o'clock and 9 o'clock).