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". Heraldic furs such as ermine and vair, and charges described as "proper", are generally exempt from the rule of tincture.
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. Earlier texts, such as the 1340 treatise De Heraudie, for example, 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 make mention of 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 Mediaeval artisans. The simplest application, the theory suggests, would be to paint a single colour over a metal shield. This practical limitation of the early Mediaeval 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 the 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 and argent, 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 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 metal field. Boutell also exempts bordures from the rule of tincture. 
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.
The rule of tincture does not apply to the claws, horns, hooves, and tongues of animals. Fox-Davies wrote that, "A lion rampant and any other beast of prey is usually represented in heraldry with the tongue and claws of a different colour from the animal. If it is not itself gules, its tongue and claws are usually represented as of that colour, unless the lion be on a field of gules. They are then represented azure, the term being 'armed and langued' of such and such a colour." Furthermore, Fox-Davies wrote that "the distinction between white and silver is marked, and a white label upon a gold lion is not metal upon metal." The same is true of white upon argent or yellow upon Or, when blazoned as such, even though argent and Or are commonly depicted as white and yellow respectively in heraldry.
Another violation which is usually not worried about is a green mount on a blue field representing the sky, and some of the methods of depicting the sea, waves or the like are similarly treated. A green trimount also appears in the coat of arms of Hungary (shown below). In this case the field is gules (red); the rule of tincture should therefore exclude this use of a vert (green) trimount. Instead, there is a trimount vert used in violation of the rule. However, it has been argued by some that the mount vert or trimount issues from the base of the shield rather than being a charge on it, causing the rule not to apply.
Marks of cadency, marks of distinction, augmentations, and abatements are also exempted from the rule. Similarly, a baronet is entitled to display a canton or inescutcheon argent charged with a hand couped gules regardless if such a mark violates the tincture rule. According to Fox-Davies, the rule of tincture also does not apply to crests or supporters, except in such cases as the crest or supporter itself is treated as a field and charged with one or more objects.
This rule of tincture is so closely followed in Great Britain and France that arms that violate the rule are called armes fausses or armes à enquérir. Likewise, in Italian heraldry violations are referred to as per inchiesta. Any violation is presumed to be an invitation to inquire how the armorial bearings came to include a violation of the tincture rule.
An example of "colour on colour" is the arms of Albania, with its two-headed eagle sable on a field gules. However, some writers in Central and Eastern European heraldry consider sable to have properties of both a metal and a colour, not exclusively a colour as it is in Western Europe, so that black-on-colour combinations are not uncommon.
This rule is perhaps most often violated by a chief, leading some commentators to question whether the rule should apply to a chief, or even whether a chief should be considered a charge at all rather than a division of the field. These violations usually occur in the case of landscape heraldry and augmentations. French civic heraldry, with its frequent chiefs of France (i.e. "Azure, three fleurs-de-lys or", anciently "Azure, semée-de-lys or"), often violates this rule when the field is of a colour. The coat of arms appearing on the famous tapestry of The Lady and the Unicorn (Paris, c.1500) was attributed until now by specialistes to the older branch and to the chief of the family Le Viste, Jean IV Le Viste, but it blatantly breaks the rules of French Heraldry. A new study of the tapestry suggests the probability of the intervention of a descendant of the younger branch, Antoine II Le Viste, as a sponsor of the tapestry, and indicates that the incorrect superposition of colours could have been a mere difference.
In French heraldry, the term cousu ("sewn") is sometimes in blazon used to get around what would otherwise be a violation of the rule; though this is used generally, occasionally a distinction is drawn between the cousu of colour on colour and the soudé ("soldered") of metal on metal, though this has fallen from fashion to a large degree.