White seems to be regarded as a different tincture from argent in Portuguese heraldry, as evidenced by the arms of municipal de Santiago do Cacém in Portugal, in which the white of the fallen Moor's clothing and the knight's horse is distinguished from the argent of the distant castle, and in the arms of the Logistical and Administrative Command of the Portuguese Air Force.
The Portuguese system of heraldic differencing is unlike any other. It has its origin in the regulations of king Manuel I.
Although it is true that the brisure (difference mark) personalizes the arms, in Portugal anyone is entitled to choose their surname (see Portuguese name) and coat of arms, which does not have to coincide with it, from whichever side of the pedigree they wish, and a system of difference marks denotes from which ancestral line the arms are derived and whether they come from parents or grandparents. The head of a lineage uses its arms without a difference; should he be the head of more than one family, the arms are combined by quartering. The heir apparent to the arms of the head of a lineage never uses a mark of difference.
Burgher arms had a complicated and suppressed history in Portugal. During the reign of King Afonso V, burgher arms were restricted to the use of colours only. This restriction would become irrelevant when King Manuel I forbade the use of arms to those who were not of the Portuguese nobility. This restriction against burgher arms in Portugal lasted until 1910.