To add a white drizzle to your red velvet macarons, simply melt a handful (about 2 oz is all you'll need) of white chocolate (you can follow this guide on how to melt chocolate in the microwave), then pour into a plastic piping bag. Let the chocolate cool slightly, then snip a very small hole in the pointy end of the bag. Drizzle the chocolate over the macarons; I like to do a criss cross pattern.