gold is a rich yellow colour, silver is white and copper is reddish.
9ct Gold is only 37% gold and the rest usually copper and silver, mixed at a rate to give a reasonalble colour.
White 9ct gold is again 37% gold with the rest mostly being silver. Some cheap stuff will however put in other white metals to keep the cost down by a few pence. so buy quality stuff.
You can get white gold 18ct gold but it is not very white as it 75% gold and 25% silver.
All the above will look a bit yellowish when put next to silver in the shop.
BUT shops (and buyers) do like their white gold extra shiney and bright white so it is often plated with another metal (Rhodium) This will wear off in time if it is on a ring but problaly last better on other stuff.
Platinum in naturally a bit grey in colour and can also be plated with Rhodium.
Platinum is only sold in a very pure state - 95% pure which you cannot do with gold as it is too soft on its own.