The different Gold Leaf options

Gold leaf comes in a variety of karat values and shades which vary from yellow to silvery. 24k gold is 99.9% gold and is yellowish in appearance.  12K is 50% gold and the rest usually silver, which results in it frequently being called White Gold.  22k gold leaf is about 92% gold.

Other metals mixed with the gold change the color or shade of the gold leaf.  More silver or palladium makes the leaf whiter.  Copper makes it redder.

22k gold leaf made with palladium instead of copper is paler and called Moon Gold. 

Pure gold leaf (24k) will not tarnish or change color over time.  Silver leaf, 18k and 12k gold leaf will darken if not sealed against oxidation.

For Fusing you can use any of the golds from 16K on up - the higher the karat the more "gold" the colour.