What metal makes up ten percent of yellow gold?

According to this jeweller's site, "yellow gold is made by mixing pure gold with alloy metals such as copper and zinc." But the percentage of each metal depends on the balance - the relative amount of gold is measured in carats. 18ct gold (the highest usually used for jewelry) contains 75% pure gold; 24 carat gold has no alloy mixed in, but is too soft for really fine jewelry - you could mash it out of shape with your bare fingers (and a fair amount of effort)! 22 carat gold would have 8.3% alloy.

http://gilletts.com.au/information.php?info_id=39

According to the diagram in this article, yellow gold is approximately 10% silver.

https://en.wikipedia.org/wiki/Gold

There is apparently no hard and fast formula for yellow gold; it seems to be more an art than a science. Various amounts of copper, silver, zinc, and other metals are added. Pure gold, of course, is naturally yellow. That would be 24 karat, 100% gold. If you're looking at gold that is 90% pure, the closest standard in 22 karat.

"24 Karat Gold Jewelry, where 24 of the 24 parts are gold, 99.99% fine gold.  22 Karat Gold Jewelry, where 22 of the 24 parts are gold and 2 parts are one or more additional metals, making it 91.7% gold."
http://goldprice.org/gold-jewelry/2005/12/24-karat-gold-jewelry.html

This says copper and silver are added:
"Yellow gold is alloyed with copper and silver for extra strength...The concentration of yellow in yellow gold is influenced by its content. The larger or purer the karat, the brighter and deeper the yellow will appear. Pure 24 karat gold, gold that has no alloys mixed in, would thus be the deepest of the colors."
http://www.since1910.com/education/precious-metals/gold-education.aspx