There are four types of teas: white, green, black, oolong (blue). In what way are they different?

Chinese tea is very diverse, but is made from the same plant, camellia sinensis. The primary difference is where the leaves are picked and processed.
Green tea is considered one of the oldest of four tea types. It is grown in the regions known as the Loess Plateau, where the soil is fertile. Green tea leaves are picked before they are fully mature, which makes them firm and green. After being picked, the leaves will continue to oxidize for some time, eventually giving rise to a tea that is fermented and slightly bitter.
Black tea is extracted from the leaves that are not picked until five days old. As the leaves oxidize, they turn a dark reddish-brown color. Because of this, the leaves must be kept moist, which enhances the color.
White tea has its origins in the mountains of Fujian province, which is where tea is traditionally made. The leaves are usually picked and oxidized in a controlled environment, then processed in a kiln. The fermentation process often causes the leaf to become a pale yellow color, with a very distinct flavor.
Oolong tea is made from the leaves that have been oxidized by using hot water. The temperature used is usually around 120 degrees Fahrenheit. The leaves are picked when they are about two days old.

https://teastoreblog.com/article/what-is-the-difference-between-green-black-white-and-oolong-tea/

It has to do primarily with the processing.
White tea leaves are the least processed ... usually just allowed to dry out and wither in direct sunlight. It's also handled in very small batches.
Green tea leaves are either steamed (Japan) or roasted (China) immediately after picking, which preserves the color and eliminates oxidation.
Black tea leaves are the most processed ... being allowed to oxidize or ferment before drying.
Oolong is similar to black tea, the difference being that the time of oxidation is much shorter.