One of the earliest rules one learns in the study of armory is that colour cannot be placed upon colour, nor metal upon metal. Now this is a definite rule which must practically always be rigidly observed. Many writers have gone so far as to say that the only case of an infraction of this rule will be found in the arms of Jerusalem: Argent, a cross potent between four crosslets or… this rule must be thrown overboard with regard to crests and supporters.
A Complete Guide to Heraldry by A. C. Fox-Davies, pg. 85