An 18 carat ring is 75% gold whether it is yellow, white or rose. The remaining quarter decides the colour. Read about the gold content itself in gold: carat and alloys.
Yellow gold
Gold mixed with silver and copper in balance. It needs the least upkeep: what you see is the metal itself, all the way through. Scratches can be polished out and the colour never changes. The higher the carat, the deeper the yellow.
Rose gold
More copper in the mix gives a pink to reddish colour. Like yellow gold it is solid colour all the way through and needs no plating. The copper makes the alloy slightly harder. People with a copper sensitivity occasionally react to it.
White gold
Gold has no white form, so white gold is gold mixed with white metals, nowadays mostly palladium. The result is not bright white but a warm grey. That is why most white gold jewellery is plated with rhodium, a bright white metal of the platinum family.
That layer wears off. After a year or a few years, depending on how the ring is worn, the warmer colour underneath begins to show, first on the underside of the band. A goldsmith can replate it. It is normal upkeep, but it is good to know beforehand. Some goldsmiths leave palladium white gold unplated on purpose: the colour is greyer but never changes.
Nickel
In the past, white gold was often alloyed with nickel, and in some parts of the world it still is. Nickel is the most common cause of jewellery allergy. In the European Union the amount of nickel that jewellery may release is restricted by law. If you have sensitive skin, ask for palladium white gold, or choose platinum.
Combining colours
Colours can be combined in one piece: a yellow band with a white setting is a classic, because white claws make a diamond look whiter. Two-colour wedding rings are a way for partners with different tastes to wear a pair that belongs together.
Which to choose
Look at what you already wear and at your skin tone, and hold the colours against your hand. For upkeep: yellow and rose gold ask nothing, rhodium-plated white gold asks for replating now and then.
