Pure (or ‘Fine’ or 24 carat) gold has a lovely warm rich yellow color that is highly prized. It is actually an orange shade of yellow. Unlike other precious metals, gold jewellery can be produced in various alloys of gold, known as the carat/karat* golds, in a range of colours from white to yellow and through to red. In addition, it is possible to achieve other special colours such as blue, black and purple. So how is it possible to change the intrinsic colour of gold? [An alloy is just a mixture of two or more metals; e.g. brass is an alloy of copper and zinc.]
The simple answer is that it is like an artist mixing his paints to obtain various hues. Every schoolboy (and girl) knows that mixing blue and yellow produces green and mixing yellow and red produces pink or rose. With metals our choices are limited, we can only mix yellow (gold) with red (copper) or white/grey (all other pure metals). So for the carat golds, we can maintain yellow at the medium and low caratages by balancing the amount of copper and silver plus zinc alloyed into the gold. If we add more copper than silver, then we get redder shades and adding more silver or other metals than copper gives us paler colours and even white.
However, we should note that, for a given caratage of gold, varying the colour also changes other properties, such as hardness and strength. We should also note that we can obtain a wider variation in colour as we lower the caratage. This is all explained in more detail in the section on gold jewellery alloys. White golds, in practice, are a little more complex since the whitening – or bleaching – effect of different metals on gold varies: see the section on white golds.
The special colors such as blue, black and purple are obtained by quite different approaches, either as special metal compounds or by surface treatments to obtain a patina. This is explained in the section on special colours.
Note: *the word ‘carat’ is spelled with a ‘k ‘ (i.e. karat) in German and in the USA. Here we use the English spelling with a ‘c’.