UIColor new property for iOS 13, such as label, secondaryLabel, tertiaryLabel and quaternaryLabel, what is use of this?
These are called Semantic Colors. That means the name of the color defines regardless of it's actual color. Instead, it's name by it's usage. This will help you to use color in dark mode and light mode and many other situations without being concerned about the contrast of the color.
Some colors (of the system) have four different variations:
These are usually lighter and darker versions of the original color.
Some of theme have other variations like, base and lifted which are used for normal and modal presentations to prevent colors from mixing visually.
Watch Implementing Dark Mode on iOS to learn more about it