I grew up in the back of my dad’s jewelry shop, always looking over the shoulder of the jewelers while they were repairing, reconditioning or manufacturing a piece from scratch.
These old-school jewelers would always throw pennies into the mix with gold to use as an alloy which I found fascinating. I remember my dad would be complaining that the color would come out too red. He would buy “real alloy mix” but the jewelers would refuse to use it. If you look at vintage jewelry from Europe, especially Italy, you can see it looks redder than what we have now. Gold has all kinds of colors. It’s hard to believe but all gold starts yellow, even white gold. The reason it looks so white is that it is usually rhodium plated. Rhodium, according to Wikipedia, is a chemical element with symbol Rh and atomic number 45. It is a rare, silvery-white, hard, corrosion-resistant and chemically inert transition metal. It is a noble metal and a member of the platinum group.
The fineness of gold (coin, bar, jewelry, etc.) represents the weight of fine gold therein, in proportion to the total weight which includes alloying base metals and any impurities. Alloy metals are added to increase hardness and durability of coins and jewelry, alter colors, and decrease the cost per weight.
The same is done with silver. For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares, and jewelry. Coin silver, which was used for making silver coins in the past, contains 90% silver and 10% copper, by mass. Sterling silver contains 92.5% silver and 7.5%, by mass, of other metals, usually copper.
Various ways of expressing fineness have been used and two remain in common use: millesimal fineness expressed in units of parts per 1,000 and karats used only for gold. Karats measure the parts per 24, so that 18 karats= 18⁄24 = 75% and 24 karat gold is considered 100% gold.
So, pure gold, the way it comes from the earth, is considered 24 karat gold. As I stated above, when being used in jewelry, it is blended with base metals to make it harder and to change the color
So, before you fall asleep with the technical jargon, pure gold, 24 karats, is a deep yellow, then it is mixed with white alloys, and it comes out looking more white than yellow, and then we plate it with rhodium to make it look basically like chrome.