Welcome to Gold School 101 :)
“Pure” gold refers to elusive 24k gold which is very rarely used for making jewelry - it is a soft metal and far too pliable. Therefore all gold jewelry is actually a mixture of metals - this is where different colors of gold come from.
Yellow gold is pure gold + silver, copper and zinc. It is the most hypoallergenic.
White gold is pure gold + platinum, or palladium, nickel and zinc. It is more scratch-resistant and stronger than yellow gold.
Rose gold is pure gold + copper and silver, which makes it the most durable of all gold colors.
The Karat of gold refers to the purity percentage of the gold and alloy mix:
18k gold refers to gold that is 75% pure, with 25% alloyed metal.
14k gold refers to gold that is 58.5% pure, with 41.5% alloyed metal (and thus a slightly lower cost than 18k).
Vermeil is gold plated sterling silver. The difference between “vermeil” and “gold plated/filled”, is that gold vermeil jewelry has a thicker gold layer and uses sterling silver as its base metal. This is why vermeil is a wiser choice for those who are allergic to nickel or other lesser base metals than other costume, gold-plated jewelry.
Gold-Plated jewelry has a very thin layer of gold on the surface of the base metal, which is usually stainless steel or brass. Gold-plating is actually a process called electro-plating. The gold layer must be at least 7 millionths of an inch thick and at least be of a quality of 10k gold or better. Because the gold layer is very thin, it can wear over time with excessive wear.