Many people associate the metal gold with the color gold. But, some gold jewelry is not golden at all: it's white.
Gold is one of the heaviest and softest metals. Pure gold, called 24-carat gold, is so soft that it can be easily dented. In the movies, old-time prospectors bite on yellow nuggets to see if they're real gold because 24-carat gold will show a bite mark. In the real world, jewelers don't want to use metal that can be dented so easily. So, most jewelry is actually an alloy of gold with harder metals.
Since pure gold is called 24-carat gold, an alloy of half gold and half other metals is called 12 carat gold, because, well, 12 is half of 24.
If most gold jewelry is an alloy, why is some white and some yellow? The final appearance depends on the type of alloy metal used. Alloys made with white metals like silver, palladium and nickel create white-colored gold. Adding some copper to the alloy will restore the yellow color, but too much copper can turn the gold reddish.
The color of gold jewelry doesn't reveal how much gold is in it a piece of white gold may have the same number of carats as a piece of yellow gold. But, it has a different alloy metal, and so it has a different color.
Unlike white gold, fool's gold has no gold in it at all. It's a mineral called iron pyrite. Black gold isn't gold either, it's oil. Which is also valuable, but doesn't look as good on your fiancée's finger.