This article is from the rec.heraldry FAQ, by Gordon Findlay (email@example.com) and Francois Velde (firstname.lastname@example.org) with numerous contributions by others.
The "colours" used on shields are strictly called tinctures; there is a
limited range which varies somewhat from place to place and time to
time. These tinctures are divided into two groups: gold and silver,
which are called the metals, and all the others, which are called the
In Woodward's words, it is a "primary heraldic canon" that colour is
not placed on colour, nor metal on metal. This rule was used to ensure
that coats of arms could be easily recognised at a distance or in the
heat of battle.
It is commonly said that the arms of Jerusalem (Argent, a cross potent
between four crosses Or) are the only counterexample. However, Woodward
quotes several examples from continental heraldry in which the "rule" is
broken: e.g. Grasse (Azure, ten stars Gules, 1, 2, 3 and 4) and Doro
(Argent, a lion Or). Augmentations of honour sometimes breach the rule
as well, and a chief of colour is often, especially in continental
heraldry, placed on top of a simpler coat, giving an appearance of
colour on colour.
What _is_ certain is that colour on colour or metal on metal is
exceedingly uncommon or non-existent in English, French and Scottish
heraldry, and that the Kings of Arms in Scotland and England would not
grant such an arrangement today.
In other countries the rule is less rigidly followed, and in some, such
as Hungary, colour on colour is very common. Most of the books in
English reflect English or Scottish heraldic practice and ignore the
heraldry of other nations.