OnPaintBackground and OnPaint overrides

protected override void OnPaintBackground(PaintEventArgs e)
{
    // do nothing
}

protected override void OnPaint(PaintEventArgs e)
{
    Color c = Color.FromArgb((int)(Opacity * 255), this.BackColor);
    e.Graphics.FillRectangle(new SolidBrush(c), this.ClientRectangle);
    base.OnPaint(e);
}