The main duty of a heraldic device is to be recognized, and the dark colours or light metals are supposed to be too difficult to distinguish if they are placed on top of other dark or light colours, particularly in poor light. Though this is the practical genesis of the rule, the rule is technical and appearance is not used in determining whether arms conform to the rule. Another reason sometimes given to justify this rule is that it was difficult to paint enamel colours over other enamel colours, or with metal over metal.