The term caratage only applies to the purity of gold alloys. So I'm assuming your work is to do with jewellery alloys and their colours.
The term carat is a measure of the amount of gold in the alloy.
one carat = 1/24 x mass of gold / total mass
So 24 carat gold is the purest being 99.9% Au (w/w). 18-carat gold is 75% Au, 12-carat gold is 50% Au. Gold is a soft, yellow (gold) coloured metal. To improve is durability it is alloyed with other metals to increase its hardness and ductility. Typically it is alloyed with copper and this also has the effect of altering the colour. So 18-carat gold is 75% Au and 25% Cu.
By adding different metals different colours of gold can be made:
Blue gold can be made by alloying with iron,
purple gold can be made by alloying with aluminium,
White gold alloys can be made by alloying with palladium or nickel.