If your app changed the color of navigation bar (background / foreground / text color / tint color), it is probably broken in the new iOS 15. Here is how I fix them in Xamarin:

if (UIDevice.CurrentDevice.CheckSystemVersion(15, 0))
{
    // change the background and text color of title bar
    var appearance = new UINavigationBarAppearance();
    appearance.ConfigureWithOpaqueBackground();
    appearance.BackgroundColor = CustomColor.AccentColor;
    appearance.TitleTextAttributes = new UIStringAttributes() { ForegroundColor = CustomColor.ForegroundColor };
    appearance.LargeTitleTextAttributes = new UIStringAttributes() { ForegroundColor = CustomColor.ForegroundColor };

    UINavigationBar.Appearance.StandardAppearance = appearance;
    UINavigationBar.Appearance.ScrollEdgeAppearance = appearance;
}
else
{
    // change the background color of title bar
    UINavigationBar.Appearance.BarTintColor = CustomColor.AccentColor;
    UINavigationBar.Appearance.Translucent = false;

    // change the title text color
    UINavigationBar.Appearance.TitleTextAttributes = new UIStringAttributes() { ForegroundColor = CustomColor.ForegroundColor };
}

// change the button color on navigation bar
UINavigationBar.Appearance.TintColor = CustomColor.ForegroundColor;