Colored golds can be classified to three groups:
Alloys with silver and copper in various proportions, producing white, yellow, green and red golds; 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
Pure 100% gold is 24 karat by definition, so all colored golds are less than this, with the common being 18K (75%), 14K (58%), and 9K (38%).