The rule of tincture

tilttek/heraldry

Metal is never laid on metal, nor colour on colour. A fur lies on either, and nothing lies on its own tincture.

use Tilttek\Heraldry\TinctureRule;

$rule = new TinctureRule();
$rule->respects($coat);                // true or false
foreach ($rule->violations($coat) as $violation) {
    $violation->describe();            // "The ordinary lies or on argent: metal on metal."
}

Where the rule binds

What	Lies on
The ordinary, a bordure included	a plain field
Charges on the field, beside an ordinary or alone	a plain field
Charges on the ordinary	the ordinary

Where it does not

Whatever lies on a divided field touches both of its tinctures: a metal and a colour field takes anything.
A mark of difference lies over everything and is not checked.

Violation names the layer (piece or charges), its tincture and the ground it lies on. Tincture::contrastsWith() is the rule for one pair.