How do I change another View's background color as same as ColorSettingView?

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There seems to be an error in Environment var declaration.

But I could not test (some struct definitions are missing. Where is Game defined ?

struct ColorSettingView: View {
    
    @Environment(\.presentationMode) var presentation // <<-- Error here: needs \.
    @State private var bgColor = Color.white

    let brPlayer = SoundPlayer()
    
    var body: some View {
        
        ZStack{
            bgColor
                .ignoresSafeArea()
            VStack {
                ColorPicker("Background Color", selection: $bgColor)
                
                Button (action: {brPlayer.brPlay()
                    self.presentation
                        .wrappedValue.dismiss()
                }, label: {
                    Image("Quit")
                        .resizable()
                        .aspectRatio(contentMode: .fit)
                        .padding(/@START_MENU_TOKEN@/[.leading, .bottom, .trailing]/@END_MENU_TOKEN@/)
                        .frame(width: 100.0, height: 100.0)
                    
                })
            }
            
        }
    }
}
                            
            
struct ColorSettingView_Previews: PreviewProvider {
    static var previews: some View {
        ColorSettingView()
    }
}

struct ContentViewX: View {
    
    let kiPlayer = SoundPlayer()
    @State private var bgColor = Color.white
    @State var ColorSettingView = false  // <<-- Not very good idea to name as the struct. Change to colorSetting
    
    var body: some View {
        
        ZStack {
            //    I also want to change this view as same as ColorSettingView
            bgColor
                .ignoresSafeArea()
            VStack {
                
                Button (action: {kiPlayer.kiPlay()
                    
                    self.ColorSettingView = true},  label: {Image("Setting")
                            .resizable()
                            .aspectRatio(contentMode: .fit)
                    }
                )
                
                .sheet(isPresented: $ColorSettingView, content: {
                    Game.ColorSettingView()}
                )
                
            }
        }
    }
}

You should declare an @ObservedObject in ContentView for the color (note that to make it observable, you may have to embed as published in an observable struct)

@ObservedObject var setting: ColorSetting()

and declare an @EnvironmentObject in ColorSettingView

    @EnvironmentObject var setting: ColorSetting

Then when you present sheet:

        .sheet(isPresented: $colorSetting) { // Changed the name
            ColorSettingView().environmentObject(setting)  // THIS IS IMPORTANT
        }

Hope that works as I could not test.

This should work. I have simplified your code, but it should be easy to reintroduce missing parts.

The point was to create an ObservableObject class and use ObservedObject and Binding

class ColorSetting : ObservableObject {
    @Published var color: Color = .white
}

struct ColorSettingView: View {
    
    @Environment(\.presentationMode) var presentation
    @Binding var bgColor : Color     // bgColor is changed here but owned by another View

    var body: some View {
        ZStack{
            bgColor
                .ignoresSafeArea()
            VStack {
                ColorPicker("Background Color", selection: $bgColor)  // We change the color, and publish the change
                
                Button (action: {
                    self.presentation
                        .wrappedValue.dismiss()
                }) {
                    Text("Quit\n\(bgColor.description)")
                        .padding()
                    
                }
            }
            
        }
    }
}
                            
struct ContentView: View {
    
    @ObservedObject var bg = ColorSetting()      // We observe it
    @State var colorSetting = false

    var body: some View {
        
        ZStack {
            //    I also want to change this view as same as ColorSettingView
            bg.color
                .ignoresSafeArea()
            VStack {
                
                Button("Show\n\(bg.color.description)") {
                    colorSetting = true
                }
                .sheet(isPresented: $colorSetting, content: {
                    ColorSettingView(bgColor: $bg.color)  // we'll get color from ColorSettingView, by Binding
                     }
                )
                
            }
        }
    }
}

There is a simpler way to do it, just with Sate and Binding

struct ColorSettingView: View {
    
    @Environment(\.presentationMode) var presentation
    @Binding var bgColor : Color

    var body: some View {
        
        ZStack{
            bgColor
                .ignoresSafeArea()
            VStack {
                ColorPicker("Background Color", selection: $bgColor)
                
                Button (action: {
                    self.presentation
                        .wrappedValue.dismiss()
                }) {
                    Text("Quit\n\(bgColor.description)")
                        .padding()
                    
                }
            }
            
        }
    }
}
                            
struct ContentView: View {
    
    @State var bg : Color = .white
    @State var colorSetting = false

    var body: some View {
        
        ZStack {
            //    I also want to change this view as same as ColorSettingView
            bg
                .ignoresSafeArea()
            VStack {
                
                Button("Show\n\(bg.description)") {
                    colorSetting = true
                }
                .sheet(isPresented: $colorSetting, content: {
                    ColorSettingView(bgColor: $bg)}
                )
                
            }
        }
    }
}

Hi, Claude31.

Thank you for your prompt reply. I tried your code. Finally It worked! I am grateful for your support!