The Colours of Gold
The colour of the gold in our charms and chains is a result of the
composition of the alloys used in the production of the 9ct gold from
which our jewellery is created.
9ct gold (375) is 9 parts pure gold and 15 parts other metals (silver, copper & a pinch of zinc) making a total of 24 parts by weight. 24 carat gold is pure gold.
Each of our manufacturers will use similar, but not identical alloys, so there will be slight differences in the colour of the gold that each one produces. As market trends change, our manufacturers might also try to create a brassier (yellower) or warmer (redder) shade of 9ct gold to suit their customers. They do this by varying the proportions of silver, copper and zinc in the alloy, whilst keeping the gold steady at 37.5%
It might be easier to picture how this works if you think of it in terms of a bucket of ice-cream containing 3 flavours (colours) as shown in the picture below.
This is a simple analogy to try to explain why not all 9ct gold is the same colour. Gold alloys can be white gold, rose gold, and all the shades of yellow gold in between. Gold can also be found in other colours such as green, grey and black. Changing the alloy will result in a change to the weight by volume.
Wikipedia has a good article.