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public void SetBackgroundColor()
{
  this.BackColor = System.Drawing.Color.Red;
}					

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 private void button2_Click(object sender, EventArgs e)
        {
            Size = this.Size;
            //Öffnen einer neuen Windows-Form            
            frm3.WindowState = FormWindowState.Maximized;
            //frm3.TopLevel = true;
            frm3.MdiParent = this;
            frm3.Show();
            //Button 1-3 unsichtbar werden lassen
            button1.Visible = false;        
            button2.Visible = false;
            button3.Visible = false;

            SetBackgroundColor(); 
            //this.SetBackgroundColor(); 
        }					

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frm3.BackColor = System.Drawing.Color.Red;