▁colour ▁of ▁nature . ▁ ▁This ▁does ▁not ▁seem ▁to ▁have ▁been ▁done ▁in ▁the ▁earliest ▁her ald ry , ▁but ▁examples ▁are ▁known ▁from ▁at ▁least ▁the ▁sevent e enth ▁century . ▁ ▁While ▁there ▁can ▁be ▁no ▁ob jection ▁to ▁the ▁occas ional ▁dep iction ▁of ▁objects ▁in ▁this ▁manner , ▁the ▁over use ▁of ▁charges ▁in ▁their ▁natural ▁colours ▁is ▁often ▁c ited ▁as ▁indic ative ▁of ▁bad ▁her ald ic ▁practice . ▁ ▁The ▁much - m aligned ▁practice ▁of ▁landscape ▁her ald ry , ▁which ▁fl our ished ▁in ▁the ▁latter ▁part ▁of ▁the ▁eigh teenth ▁and ▁early ▁part ▁of ▁the ▁nin ete enth ▁century , ▁made ▁extensive ▁use ▁of ▁such ▁non - her ald ic ▁colours . ▁ ▁One ▁of ▁the ▁most ▁important ▁convent ions ▁of ▁her ald ry ▁is ▁the ▁so - called ▁" rule ▁of ▁t inct ure ". ▁ ▁To ▁provide ▁for ▁contrast ▁and ▁visibility , ▁met als ▁should ▁never ▁be ▁placed ▁on ▁met als , ▁and ▁colours ▁should ▁never ▁be ▁placed ▁on ▁colours . ▁ ▁This ▁rule ▁does ▁not ▁apply ▁to ▁charges ▁which ▁cross ▁a ▁division ▁of ▁the ▁field , ▁which ▁is ▁partly ▁metal ▁and ▁partly ▁colour ; ▁nor , ▁strictly ▁speaking , ▁does ▁it ▁prevent ▁a ▁field ▁from ▁consisting ▁of ▁two ▁met als ▁or ▁two ▁colours , ▁although ▁this ▁is ▁unusual . ▁ ▁F urs ▁are ▁considered ▁am ph ib ious , ▁and ▁neither ▁metal ▁nor ▁colour ; ▁but ▁in ▁practice ▁er mine ▁and ▁er min ois ▁are ▁usually ▁treated ▁as ▁met