Melt the white chocolate in a double boiler. In a medium bowl, whisk together the confectioners' sugar, corn syrup and heavy cream until smooth, then stir into the white chocolate until combined. Turn the heat off and leave the chocolate over the double boiler, whisking occasionally to keep smooth. If the glaze is too thick, add more cream and whisk until smooth.
Divide the glaze between two bowls. With food coloring, dye each bowl a separate color (whichever you choose). Add the second bowl of colored glaze back to the first, pouring only along the edge. Use a skewer to swirl only slightly (when you dip your donuts you will swirl the glaze more).