The first and foremost rule of painting planets is to select the right color of the lights. You can't just grab a random color from the rainbow box and call it a planet. There are two methods to select a color, one is to paint the light from the sun or the moon, or you can paint it from the atmosphere of the planet. The second method is less accurate, but it will render an illusion to the eyes. The first method, as mentioned above is more accurate, will give you a better result, but is time consuming. Also, you have to be careful, as there are some things to keep in mind while choosing the color. If the atmosphere is pure white, the planet will appear blue. On the other hand, if the atmosphere is really thick and has heavy particles, the planet will appear orange or red. This method is also helpful in choosing the right color to paint the clouds.