private void button1_Click(object sender, EventArgs e)
{
    panel1.BorderStyle = BorderStyle.None;
    panel1.BackColor = Color.Blue;
    GraphicsPath vGraphicsPath = new GraphicsPath();
    vGraphicsPath.AddEllipse(new Rectangle(0, 0, panel1.Width, panel1.Height));
    vGraphicsPath.AddEllipse(new Rectangle(10, 10, 
        panel1.Width - 20, panel1.Height - 20));
    panel1.Region = new Region(vGraphicsPath);
}