Pure (24 carat) gold is bright yellow and very soft, and for these reasons is rarely used for jewellery. Instead, we mix the gold with other metals to make a harder alloy and, depending on the quantity and nature of the other metals, we achieve different colours and carats (see Carat System for an explanation of how this works). The most commonly used metals are copper, silver bronze and palladium. An alloy of gold, copper and bronze gives us the traditional yellow shade, one of gold and copper creates red gold and one of gold, silver and palladium gives white gold.
Being aware that gold mining is not very environmentally-friendly, I have decided only to use recycled gold. I purify to 24 carats and then I make my own alloys. I only use 18 carat gold and the alloy I prefer is gold with pure silver and a trace of copper (for hardness, rather than colour), which gives a very distinctive pale yellow gold known as "green gold". Another favourite of mine is an alloy of palladium and silver that gives a characteristic grey-white gold colour. I love playing with different shades of gold and that's the reason I always experiment with different alloys for future collections.
My silver is 925 (called sterling silver in the UK, which is 92.5% pure) and 999 silver (99.9% pure) for the parts encasing the stones. I also try to recycle silver as much as possible.