You can absolutely add in dark or white chocolate for the homemade turtle candy. Using dark chocolate will change the turtle candy slightly, since the flavor profile of dark chocolate is a bit more bitter than milk chocolate. White chocolate, since the flavor is different from regular chocolate, is going to be a delicious twist to the homemade chocolate turtle candy. You could even simply cover it with milk chocolate and then drizzle it with a bit of melted white chocolate for a fun combination.