g.DrawEllipse(
new Pen(getRandomColor(),2),
x0 - r, y0 - r, r * 2, r * 2
);

Random r = new Random();
        Color getRandomColor()
        {
            return Color.FromArgb(
                 r.Next(256),
                 r.Next(256),
                r.Next(256));

        }

private void button1_Click_1(object sender, EventArgs e)
        {
            Graphics g = this.CreateGraphics();
            int x0 = this.Width / 2;
            int y0 = this.Height / 2;
            for(int r = 0; r < this.Height / 2; r++)
            {
                g.DrawEllipse(
                    new Pen(getRandomColor(),2),
                    x0 - r, y0 - r, r * 2, r * 2
                    );
                    
            }
            g.Dispose();//释放资源

        }