Gold itself is bright yellow color but can be mixed with other metals to give different colors.

Yellow Gold 18K is made of 75% Gold, 10-20% Silver and 5-15% Copper while Yellow Gold 22k has 92% Gold, 4.2% Silver and 4.2% Copper. 22K is the purist Gold that you can get. On the other hand, White Gold 18K has 75% Gold, 18.5% Silver, 1% Copper and 5.5% Zinc.

You can find out the composition of other type of Gold here.