In its native state, Gold is always a golden-yellow and is way too soft to wear.  That being said, gold is usually alloyed with other metals to increase its hardness for use in jewelry.   While there are many color varieties of gold on the market, rose, white, or green gold do not occur naturally, but are a result of alloying yellow gold with various other metals (copper, silver, zinc, nickel, platinum, palladium).

Pale or white gold results when either silver, platinum, nickel, zinc, or palladium are added.  White gold is actually "gray-white" almost steel-like in color and was originally developed to imitate platinum at a fraction of its price and is substantially harder than yellow gold. In the United States, nickel based white gold alloys are predominantly used by the jewelry industry. In Europe, both nickel and palladium based white gold alloys are used.  Most skin irritations caused by wearing white gold occur due to the nickel content.   If you like favor the look of white gold, be sure to ask your jeweler if the white gold is a nickel based alloy to prevent any potential allergic reactions.

 Adding copper yields rose, red, or pink gold and iron gives gold a blue tinge.  Blue gold is an alloy of between 18K to 23K yellow that contains the remainder karat weight in small amounts of iron and nickel.  The purity of alloy gold is expressed as its karat value, defined as the proportion of pure gold metal present in the alloy.  Therefore, 24K gold is pure gold (actually 99.99 percent as one cannot guarantee an absolute purity), 12K gold is 50% purity, 18K gold is 75% purity.