Delegate function in appDelegate doesn't change navbar colors

I'd suggest, since you want to dynamically change the NavigationBar theme (like background color) based on a Flag model value, to not go for the AppDelegate way, as that will do it once for you, and it is thought more as a global way to set the NavigationBar style before any view is actually created.

There are a few ways you can apply that, like through extension ViewController, inheritance with base class.. and as well different ways you can get/set the flag values to change the navigation colors, like through userdefaults, variables... I'll show an example just to get you going:

import UIKit

class ViewController: BaseViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        currentFlag = "Canada"
    }
}

class BaseViewController: UIViewController {

    var currentFlag: String = "General" {
        didSet {
            setNavBarColor()
        }
    }

    private let themeColorUS = UIColor(red: 0.991, green: 0.621, blue: 0.022, alpha: 1.00)
    private let themeColorCanada = UIColor(red: 0.001, green: 0.686, blue: 0.000, alpha: 1.00)
    private let themeColorGeneral = UIColor(red: 0.000, green: 0.954, blue: 0.969, alpha: 1.00)

    override func viewDidLoad() {
        super.viewDidLoad()

        setNavBarColor()
    }

    private func setNavBarColor() {
        navigationController?.navigationBar.barTintColor = getBarColor(for: currentFlag)
    }

    private func getBarColor(for flag: String) -> UIColor {
        if flag == "US" {
            return themeColorUS
        } else if flag == "Canada" {
            return themeColorCanada
        }

        return themeColorGeneral
    }
}

And that means, we removed the global way of setting its style from AppDelegate so my didFinishLaunchingWithOptions looks like:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    return true
}

Running the following code (with flag set to Canada for my ViewController), and having ViewController in storyboard with a root viewcontroller of UINavigationController like this:

Would make the app look like:

Refactoring for improvements

Something you can do in addition, just to make it easier to manage the code and flags and colors, is to organise them in a structure, I choosed an enum as an example but you can do it other ways as well, just to give you a sample you could have done it this way:

import UIKit

enum Flag {
    case us
    case canada
    case general

    static let `default` = Flag.general

    init(rawValue: String) {
        switch rawValue {
        case "US":
            self = .us
        case "Canada":
            self = .canada
        case "General":
            self = .general
        default:
            self = .default
        }
    }

    var themeColor: UIColor {
        switch self {
        case .us:
            return UIColor(red: 0.001, green: 0.686, blue: 0.000, alpha: 1.00)
        case .canada:
            return UIColor(red: 0.001, green: 0.686, blue: 0.000, alpha: 1.00)
        case .general:
            return UIColor(red: 0.000, green: 0.954, blue: 0.969, alpha: 1.00)
        }
    }
}

class ViewController: BaseViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        currentFlag = .canada
    }
}

class BaseViewController: UIViewController {

    var currentFlag: Flag = .default {
        didSet {
            setNavBarColor()
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        setNavBarColor()
    }

    private func setNavBarColor() {
        navigationController?.navigationBar.barTintColor = currentFlag.themeColor
    }
}