Following the classical Greek analysis of light and colour, black and white were considered the two primaries since the interplay between light and dark is what was held to produce the spectrum. Furthermore, white, or more accurately, light, was not defined as a colour or ‘tincture’ but as the gold or the silver which, to this day, are still the only options for the term ‘metal’ in the language of heraldry. Black, therefore, was considered the most important of colours, ranking above the red, blue and green standardly referred to as ‘tinctures’.
Thirteenth century texts explaining the imperial insignia go even further. Because of medieval conceptions of the absorption of light by darkness, the writers theorized that within the color black was contained all the light or the white it had displaced.
This is obviously the reason why when the ruby is substituted for red or ‘gules’ and the emerald for green or ‘vert’ according to the traditions of gemnological blazonry, it is nothing other than the diamond that stands for ‘sable’. In all probability, it is also this line of reasoning that contributed to the cult of the Black Madonna. For, having borne the Light of Creation within her very womb, the devotion to the Mother of God as the (coal) black Queen of Heaven is a superb example of how this law of physics was at one time interpreted.