Dynamically add button and its event to WinForms

Assuming you want the Button to continue switching colors with every Click, and that you do not need to preserve the original Form BackColor on start-up:

Color firstColor = Color.Red;
Color secondColor = Color.Green

bool whichColor = true;

private void btn_Click(Object s, EventArgs e)
{
    BackColor = whichColor ? firstColor : secondColor;

    whichColor = ! whichColor;
}

Or, you could simplify this to:

private void button1_Click(object sender, EventArgs e)
{
    BackColor = (whichColor = ! whichColor) ? firstColor : secondColor;
}

You could simplify even further if you are sure you don't need access to the created Button outside the scope/context in which it was created at run-time:

private void SomeForm_Load(object sender, EventArgs e)
{
    // outside of the Load Event you will have no access
    // to the Button you create here !
    Button newButton = new Button();
    
    newButton.AutoSize = true;
    newButton.Location = new Point(400, 100);
    newButton.Text = "Click to Change Form Background Color";
    newButton.ForeColor = Color.White;
    
    // anonymous EventHandler defined with Lambda expression
    newButton.Click += (obj, eArgs) =>
    {
        BackColor = (BackColor == Color.Red) ? Color.Green: Color.Red;
    };
    
    Controls.Add(newButton);
}

Note the interesting fact that if the Control you create, as a child Control of a Form, does not have its BackColor, set when you create it at run-time, it will "inherit" the BackGround Color of the Form, and as the form BackColor is changed, the Button will change BackGround Color also: I have never seen this behavior "officially" described by Microsoft !

But, that behavior was exactly why I set the ForeColor property of the Button to white: so that it would be visible whether the Button BackColor was red, or green :)