The rule of tincture is a design philosophy found in some heraldic traditions that states "metal should not be put on metal, nor colour on colour". The rule has been widely adopted by heraldic authorities and has been described as 'definite' and 'practically unbreakable'. However, the rule has also over time been significantly read-down, both by a narrowing of that to which it applies, and by the adoption of many exceptions. Thus, heraldic furs (such as ermine and vair), and animals and other charges represented "proper" (in their natural colours), are generally exempt from the rule of tincture. Certain elements of full heraldic achievements (for example, crests, supporters), and a variety of components in coats of arms (for example, bordures or animals' claws), are not covered by the rule.

Speculation about a rule regarding the use of tinctures first appeared in the Argentaye tract, a heraldic treatise dated to 1410. The Liber Armorum, dated to c. 1440, and the Blason des Couleurs, dated to c. 1440–1450, also mention the rule of tincture. This rule was purely conjectural on the heraldists' part. An earlier text, the 1340 treatise De Heraudie, makes no mention of the rule, even when discussing armorial bearings that violate the supposed rule. Other 14th-century authors, like Bartolo and Johannes de Baudo Aureo, do not mention the rule either.

A modern hypothesis argues that it was simply impractical to paint colour over colour and inlay metals in metals with the tools available to early medieval artisans. The simplest application, the theory suggests, would be to paint a single colour over a metal shield. This practical limitation of the early medieval period was misinterpreted three centuries later as the so-called rule of tincture.

Regardless, by the late 17th century the rule of tincture had gained a foothold in many countries, and was proving particularly popular in Great Britain and France. By the Victorian era, the rule of tincture was considered de facto heraldic law in England, but strict adherence to the rule proved to be impractical as arms became more complicated. Victorian heralds devised several technical exemptions to the rule of tincture during this period; these dispensations survive to this day as "lawful exemptions" to the rule.

In modern times, the rule of tincture has been adopted by virtually all heraldic authorities and societies.

Proponents of the rule of tincture argue that the main duty of a coat of arms is to be easily recognisable, and that certain tincture pairs are difficult to distinguish when placed atop or over each other. Critics argue that the exceptions are so numerous that the rule is virtually meaningless. The rule of tincture as described by Humphrey Llwyd in 1568 states that "metal should not be put on metal, nor colour on colour". The heraldic metals are or (gold) and argent (silver), and the colours are sable, gules, azure, vert, and purpure. The stains are considered colours for the purposes of the rule. The rule of tincture does not apply to furs, nor to charges that are displayed in their natural tinctures and blazoned "proper". The rule of tincture also does not apply when a charge is composed of both a colour and metal, and can be placed on a field of either a colour or metal.

Simple divisions of the field are considered to be beside each other, not one on top of the other, so the rule of tincture does not apply. A field party or patterned of a colour and a metal may have a charge of either colour or metal placed upon it. Likewise, a charged divided party or patterned of a colour and metal may be placed on either a colour or a metal field.

Fimbriation, the surrounding of a charge by a thin border, is often used to circumvent what would otherwise be a violation of the rule. In French heraldry a divise is a thin band added just beneath a chief to prevent violations, which is similar to the fillet in English heraldry. Boutell exempts bordures from the rule of tincture.