Rule of tincture

The first rule of heraldic design is the rule of tincture: metal should not be put on metal, nor colour on colour (Humphrey Llwyd, 1568). This means that Or and argent (gold and silver, which are represented by yellow and white) may not be placed on each other; nor may any of the colours (i.e. azure, gules, sable, vert and purpure) be placed on another colour. Heraldic furs (i.e. ermine, vair and their variants) as well as “proper” (a charge coloured as it normally is in nature) are exceptions to the rule of tincture. […]

Lawful exceptions

The rule of tincture does not apply to furs, nor to charges blazoned “proper” (displayed in their natural colour, which need not be a heraldic tincture).^[1] The blazoning of a charge “proper” can therefore be used as a loophole when its natural coloration equates to or approaches another heraldic tincture it is desired to overlie. An example would be a white horse proper, since without breaking the rule of no metal on metal it could be placed on a field Or, but a horse argent, although visually indistinguishable, could not. […]

Fimbriation, the surrounding of a charge by a thin border, can obviate what would otherwise be a violation of the rule, as in the Union Jack (which, although a flag rather than a shield, was designed using heraldic principles). The divise, a thin band running underneath the chief in French heraldry, can also obviate a violation, as can the parallel fillet in English heraldry. […]

Violations

This rule is so closely followed that arms that violate it are called armes fausses (false arms) or armes a enquerir (arms of enquiry); any violation is presumed to be intentional, to invite the viewer to ask how it came to pass.

One of the most famous armes a enquerir (often erroneously said to be the only example) was the arms chosen by Godfrey of Bouillon,^[7] and later used by his brother Baldwin of Boulogne when he was made King of Jerusalem, which had five gold crosses on a silver field (traditionally rendered “Argent a Cross potent between four plain Crosslets Or”). This use of metal on metal is seen on the arms of the King of Jerusalem, the Bishop’s mitre in the arms of Andorra, and the arms of the county of Nord-Trondelag in Norway (which is based on the arms of St. Olav as described in the sagas of Snorri). It indicates the exceptional holy and special status of this particular coat of arms.