Soooo...it's pinkpure gold is too soft to use for jewelry and the like. So, it's alloyed with other metals: silver, copper, palladium, rhodium, etc. The various colored golds have
18K Red gold: 75% gold, 25% copper
18K Rose gold: 75% gold, 22.25% copper, 2.75% silver
18K Pink gold: 75% gold, 20% copper, 5% silver[3]
so, you've got a standardized alloy that you can attempt to match with anodized aluminum.