Here is another robot that can decorate balls, eggs, and other spherical objects. The Sphere-o-bot is inspired by EggBot from Evil Mad Scientist. This 2-axis drawing machine is adjustable and has 3D printed components. You are going to need a SG90 servo, Arduino Leonardo board, JJRobots brain shield, 2 stepper motor drivers, and a few other parts.
Access to a 3D printer is ideal. You can program the robot in Arduino IDE. More info is available on Instructables.