How to change app tint color (New SwiftUI life cycle app)?

In the SceneDelegate.swift where you create the window for your app you can set the tint color globally using the tintColor property of UIWindow

let contentView = ContentView()

if let windowScene = scene as? UIWindowScene {
    let window = UIWindow(windowScene: windowScene)
    window.rootViewController = UIHostingController(rootView: contentView)
    self.window = window

    self.window?.tintColor = UIColor.red // Or any other color you want
    window.makeKeyAndVisible()
}

Edit
After seeing that you want it for the new SwiftUI, you can create new EnvironmentKeys:

private struct TintKey: EnvironmentKey {
    static let defaultValue: Color = Color.blue
}

extension EnvironmentValues {
    var tintColor: Color {
        get { self[TintKey.self] }
        set { self[TintKey.self] = newValue }
    }
}
   
@main
struct YourApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView().environment(\.tintColor, Color.red)
        }
    }
}

Then in your views you would use it like this:

struct ContentView: View {
    @Environment(\.tintColor) var tintColor
    
    var body: some View {
        VStack {
            Text("Hello, world!")
                .padding()
            Button(action: {}, label: {
                Text("Button")
            })
        }.accentColor(tintColor)       
    }
}

This works without an EnvironmentKey, and propagates to all views in the app:

@main

struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
                .accentColor(.red) // pick your own color
        }
    }
}

Ariel, In this implementation, you can choose and use 'accentColor' throughout the entire application lifecycle. However, if you want to keep a value permanently, you should think about a solution, I hope you are a smart guy ...

 enum MyColor: Identifiable, CaseIterable {
    var id: String { UUID().uuidString }
    case blue, green, orange, pink, purple, red, yellow
    var currentColor: Color {
        switch self {
        case .blue:
            return .blue
        case .green:
            return .green
        case .orange:
            return .orange
        case .pink:
            return .pink
        case .purple:
            return .purple
        case .red:
            return .red
        case .yellow:
            return .yellow
        }
    }
}

final class ViewModel: ObservableObject {
    @Published
    var isPresented = false
    @Published
    var myColor: MyColor = .blue
}

struct AppSettings: View {
    @ObservedObject
    var vm: ViewModel
    var body: some View {
        NavigationView {
            Form {
                Picker("Current color", selection: $vm.myColor) {
                    ForEach(MyColor.allCases) { color in
                        Label(
                            title: {
                                Text(color.currentColor.description.capitalized)
                            }, icon: {
                                Image(systemName: "circle.fill")
                            })
                            .tag(color)
                            .foregroundColor(color.currentColor)
                    }
                }
            }
            .navigationBarItems(
                trailing:
                    Button(
                        action: {
                            vm.isPresented.toggle()},
                        label: {
                Text("Close")
            }))
            .navigationTitle("App settings")
        }
    }
}

struct ContentView: View {
    @StateObject
    private var vm = ViewModel()
    var body: some View {
        VStack {
            Button(
                action: {
                    vm.isPresented.toggle()
                }, label: {
                    VStack {
                        Rectangle()
                            .fill(Color.accentColor)
                            .frame(width: 100, height: 100)
                        Text("Settings")
                            .font(.title)
                    }
                })
        }
        .accentColor(vm.myColor.currentColor)
        .sheet(
            isPresented: $vm.isPresented) {
            AppSettings(vm: vm)
                .accentColor(vm.myColor.currentColor)
        }
    }
}