This is a fun trick that uses only glaze and lots of creativity. These cookies are so much fun to make. I prefer to use the Powdered Sugar Glaze for this technique, rather than the thinned down Meringue Powder Buttercream that I usually use. It is simpler to make and works better for these little tricks.
This is a wet-on-wet technique. You will need to make glaze in all the colors that you want for your cookies. Leave some of the glaze in bowls for glazing the cookies and put some in bags for piping the decorations. Because you will be piping with glaze, you don't really need a decorating tip. I recommend using zip-top sandwich bags as your decorating bag (see Decorating Bags: Preparing). Scoop some of the prepared glaze into a bag (one bag for each color), zip up the tops, and snip off a corner of each bag to squeeze the glaze through. Glaze the cookies as directed in the glazing tutorial. While the glaze is still wet, squeeze some of the glaze from the bags onto the cookie. Use the pointy end of your cookie scraper or a toothpick to swirl, marble, or feather your designs. See examples below.