[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern IntPtr GetWindowDC(IntPtr hwnd);
[...]
System.Drawing.Graphics g = System.Drawing.Graphics.FromHdc(GetWindowDC(base.Handle));
g.FillRectangle(System.Drawing.Brushes.Red, new System.Drawing.Rectangle(60, 4, 100, 100));
g.Dispose();