Colleges of Arms came into being to resolve conflicting claims on a device, and the heralds gradually assumed the job of keeping track of who owned which device. In the S.C.A., we have our own College of Arms to insure that each member has his or her own unique device. Ideally, the result is simple, memorable, and easily identified. To help you create a good device, the heralds restrict the possible colors, poses, and arrangements of charges in your device. Nor do the heralds care what artistic style you draw your device in, as long as the style was used in the heraldry of some place or time.
Good devices have as few charges and colors as possible.
More often, you'll have to use more than one kind of charge, or more than two colors. Good devices repeat themselves. If the same kind of charge appears three places, all three are identical. Good devices make it easy to identify each charge. Other charges are drawn to make them as distinctive as possible.