Hi Matthias,

Assembling various code bits from the net the I've created a
TransparentPanel class that may work for you.  

public class TransparentPanel : Panel
{
   private int _alfa;

   public int Alfa
   {
       get { return _alfa; }
       set { _alfa = value; }
   }

   public TransparentPanel()
   {
       SetStyle(ControlStyles.Opaque, true);
   }

   protected override CreateParams CreateParams
   {
       get
       {
           CreateParams createParams = base.CreateParams;
           createParams.ExStyle |= 0x00000020;
           return createParams;
       }
   }

   protected override void OnPaint(PaintEventArgs e)
   {
       using (SolidBrush brush = new SolidBrush(Color.FromArgb(Alfa,
BackColor)))
       {
           e.Graphics.FillRectangle(brush, ClientRectangle);
       }
   }
}

It does not appear to like being tweaked in the designerwindow much, but
when adding it manually it does display the background in a faded manner.

   public Form1()
   {
       InitializeComponent();

       Controls.Add(new TextBox());

       TransparentPanel p = new TransparentPanel();
       p.Alfa = 64;
       p.BackColor = Color.DarkOrange;
       Controls.Add(p);
       p.BringToFront();
   }