I've study the solution from jan'21 where the buttoncolor change on click. My goal is to do this with multiple buttons but on click all the buttons change color. This is my code:

struct ContentView: View {

let buttons = [
["7", "8", "9"],
["4", "5", "6"],
["1", "2", "3"]
]

@State private var number: Int = 0
@State private var colors:[Int] = [0, 0, 0, 0]

var body: some View {
    Self._printChanges()
    return VStack {
        ForEach(buttons, id: \.self) { row in

        HStack {
                ForEach(row, id: \.self) { button in
                    Button("\(button)") {
                        changeColor()
                    }
                    .font(.system(size: 32))
                    .frame(width: 80, height: 80)
                    .buttonStyle(ChangeColorButton(number: number))
                }
            }
        }
    }
}

func changeColor() {
    if number == 3 {
        number = 0
    } else {
        number += 1
    }
}

struct ChangeColorButton: ButtonStyle {
    let colors: [Color] = [.gray, .green, .red, .yellow]
    let number: Int

    func makeBody(configuration: Configuration) -> some View {
        configuration.label
            .frame(width: 25, height: 25, alignment: .center)
            .padding()
            .background(colors[number])
    }
}

}

My goal is to do this with multiple buttons but on click all the buttons change color.

Yes, because they are all using the same variable (number) to determine the color:

.buttonStyle(ChangeColorButton(number: number))

I'm not entirely clear what it is you are trying to achieve, but if you want the color of the Buttons to be independent of one another, you need to track their state independently.

Something like this, perhaps:

struct ColoredButtons: View {
    let buttons = [
        ["7", "8", "9"],
        ["4", "5", "6"],
        ["1", "2", "3"]
    ]

    @State private var colors = [
        [0, 0, 0],
        [0, 0, 0],
        [0, 0, 0]
    ]

    var body: some View {
        let _ = Self._printChanges()
        VStack {
            ForEach(buttons.indices, id: \.self) { row in
                HStack {
                    ForEach(buttons[row].indices, id: \.self) { col in
                        Button("\(buttons[row][col])") {
                            changeColor(row, col)
                        }
                        .font(.system(size: 32))
                        .frame(width: 80, height: 80)
                        .buttonStyle(ChangeColorButton(number: colors[row][col]))
                    }
                }
            }
        }
    }

    func changeColor(_ row: Int, _ col: Int) {
        let currentColor = colors[row][col]
        if currentColor == 3 {
            colors[row][col] = 0
        } else {
            colors[row][col] = currentColor + 1
        }
    }

    struct ChangeColorButton: ButtonStyle {
        let colors: [Color] = [.gray, .green, .red, .yellow]
        let number: Int

        func makeBody(configuration: Configuration) -> some View {
            configuration.label
                .frame(width: 25, height: 25, alignment: .center)
                .padding()
                .background(colors[number])
        }
    }
}

Thank you @roosterboy for your solution. I was struggle for months now to get this right but now I can go further with it. Greetings @PaddyTurtle