Changing navigation bar color in Swift

Navigation Bar:

navigationController?.navigationBar.barTintColor = UIColor.green

Replace greenColor with whatever UIColor you want, you can use an RGB too if you prefer.

Navigation Bar Text:

navigationController?.navigationBar.titleTextAttributes = [.foregroundColor: UIColor.orange]

Replace orangeColor with whatever color you like.

Tab Bar:

tabBarController?.tabBar.barTintColor = UIColor.brown

Tab Bar Text:

tabBarController?.tabBar.tintColor = UIColor.yellow

On the last two, replace brownColor and yellowColor with the color of your choice.

Here are some very basic appearance customization that you can apply app wide:

UINavigationBar.appearance().backgroundColor = UIColor.greenColor()
UIBarButtonItem.appearance().tintColor = UIColor.magentaColor()
//Since iOS 7.0 UITextAttributeTextColor was replaced by NSForegroundColorAttributeName
UINavigationBar.appearance().titleTextAttributes = [UITextAttributeTextColor: UIColor.blueColor()]
UITabBar.appearance().backgroundColor = UIColor.yellowColor();

Swift 5.4.2:

UINavigationBar.appearance().backgroundColor = .green // backgorund color with gradient
// or
UINavigationBar.appearance().barTintColor = .green  // solid color
    
UIBarButtonItem.appearance().tintColor = .magenta
UINavigationBar.appearance().titleTextAttributes = [NSAttributedString.Key.foregroundColor : UIColor.blue]
UITabBar.appearance().barTintColor = .yellow

More about UIAppearance API in Swift you can read here.