// Calculate number of colors in current image
procedure TMainForm.miCalccolornumberClick(Sender: TObject);
var
  nc: integer;
begin
  if ImageEnView1.CurrentLayer.Kind <> ielkImage then
    ShowMessage('An image layer is not selected')
  else
  begin
    nc := ImageEnView1.Proc.CalcImageNumColors;
    ShowMessage('The active images has ' + inttostr(nc) + ' colors.');
  end;
end;

var OldActiveLayer: Integer;
OldActiveLayer := ImageEnView1.LayersCurrent;
try
  // if you only want to get the number of colors in your TImageEnView image
  ImageEnView1.LayersCurrent := 0;
  nc := ImageEnView1.Proc.CalcImageNumColors;
finally
  ImageEnView1.LayersCurrent := OldActiveLayer;
end;