function GetColorDepth: string; 
var 
  DesktopDC: HDC; // LongWord 
  BitsPerPixel: Integer; 
begin 
  DesktopDC    := GetDC(0); 
  BitsPerPixel := GetDeviceCaps(DesktopDC, BITSPIXEL); 
  case BitsPerPixel of 
    4: Result  := '4-Bit'; 
    8: Result  := '8-Bit'; 
    16: Result := '16-Bit'; 
    24: Result := '24-Bit'; 
    32: Result := '32-Bit'; 
  end; 
  ReleaseDC(0, DesktopDC); 
end; 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
  ShowMessage(GetColorDepth); 
end;