What is the difference between 18 and 9 carat gold?

Gold is a pure metal but is inherently soft, therefore to make practical jewellery it must be alloyed with other metals to make it harder. Pure gold is known as 24 carat or 1000 parts pure in practice 999, 18 carat gold is a high purity gold alloy of 750 parts per 1000 (75%), 9 carat gold is the lowest UK legal standard at 375 parts per 1000 (37.5%). Alloys which fall below this standard cannot legally be described as gold, though some countries do allow lower standards.   

I thought gold was yellow so what is red (rose) or white gold?

Pure gold is yellow but as gold used in jewellery production is an alloy (see previous question) it is possible to change the colour balance by adding other metals. White gold may contain proportions of Palladium, Nickel and Silver. Red gold contains proportions of  copper. It is worth noting that gold and copper are the only two coloured metals, all other metals are shades of grey grey/white. The higher the carat of gold the less colour change is possible, therefore 18 carat gold will always be more yellow than 9 carat gold.

Why has my white gold ring dulled over time?

Normal wear and tear will result in fine surface scratches building up over time which will reduce the shine of all metals. With white gold however, it is inevitable that the ring would have been covered with a rhodium plated finish for that 'showroom look'. As this is a thin electroplated finish it will deteriorate over time, especially in high friction areas for example on the setting or where it abuts another ring.

What is rhodium and rhodium plating?

Rhodium is a precious metal in the same group as platinum and palladium and is the whitest of all metals. Electroplating will deposit a thin layer (measured in microns) of metal which will wear away to reveal the host metal beneath. In jewellery production it is used extensively to make white gold jewellery look whiter and on cheap mass manufactured jewellery to cover yellow gold jewellery to make it look white especially on one part castings around the settings. It is rare to find a white gold ring on the high street which has not been rhodium plated. Unfortunately not all retailers tell their customers this, and incredibly some do not know the jewellery has been rhodium plated or even know about this process.