The basic inbox gestures still involve swiping right or left on an email to reveal tappable actions that you can perform instantly, without having to call up additional menus.
One of the default options that appear is the Flag action, which you might use to categorize a message that requests information needed by a certain date, for example.
Using only the swipe gesture, you'd be forgiven for thinking that the Mail app provides only one color to use when flagging emails, but iOS 13 actually introduces support for multicolor flags – it's just hidden away in the menu that appears when you hit the Reply button.
Tap the Flag button there, and you'll reveal a submenu that allows you to choose one of seven colors, including the option to remove a flag. Note that whichever color you select here subsequently becomes the default color when you tap the Flag action or the More -> Mark... option via the inbox swipe gesture.
Did you know that you can customize the actions that appear when you use the Mail app's inbox gestures? Click here to learn how.