Pure gold is slightly reddish yellow in color, but colored gold in various other colors can be produced by alloying gold with other elements.
Colored golds can be classified in three groups:
Alloys with silver and copper in various proportions, producing white, yellow, green and red golds. These are typically malleable alloys.
Intermetallic compounds, producing blue and purple golds, as well as other colors. These are typically brittle, but can be used as gems and inlays.
Surface treatments, such as oxide layers.