private void button2_Click(object sender, EventArgs e)
{
InvokePaintBackground( );
this.Hide( );
this.Visible=true;
}
protected override void OnPaintBackground(PaintEventArgs e)
        {
            int y, dy;
            y = this.ClientRectangle.Location.Y;
            dy = this.ClientRectangle.Height / 256;
            for (int i = 255; i >= 0; i--)
            {
                Color c = new Color();
                c = Color.FromArgb(Convert.ToInt32(textBox1.Text.ToString()), i, Convert.ToInt32(textBox2.Text.ToString()));
                SolidBrush sb = new SolidBrush(c);
                Pen p = new Pen(sb, 1);
                e.Graphics.DrawRectangle(p, this.ClientRectangle.X, y, this.Width, y + dy);
                y = y + dy;
            }
        }