procedure TForm1.Button1Click(Sender: TObject);
var DesktopDC    : HDC;
    BitsPerPixel : integer;
begin
   DesktopDC := GetDC(0);   // Device-Context des Desktops
   BitsPerPixel := GetDeviceCaps(DesktopDC, BITSPIXEL);
   case BitsPerPixel of
     4: ShowMessage('16 Colors (4-Bit)');
     8: ShowMessage('256 Colors (8-Bit)');
    16: ShowMessage('64K Colors (16-Bit)');
    24: ShowMessage('16M Colors (24-Bit)');
    32: ShowMessage('True Color (32-Bit)');
   end;
   ReleaseDC(0, DesktopDC);
end;