Lay the sugar cookies on a work surface. Pipe a line of pink icing around the edge of each of 4 cookies, then flood the centers with the pink icing. Immediately pipe a circle of white icing just inside each pink icing line. Pipe a dot of white icing in the center of each cookie. Drag a toothpick through the white icing traveling toward the center of the cookie. Repeat the process in 1-inch intervals, creating citrus segments. Repeat this process using the orange, green and yellow icings on the remaining cookies. Let stand until set, about 1 hour.