The following example demonstrates converting a TColor to a RGB color, and formating the RGB color to be shown as hexadecimal digits.

procedure TForm .Button Click(Sender: TObject);
var
  TheRgbValue : TColorRef;
begin
  if ColorDialog .Execute then begin
    TheRgbValue := ColorToRGB(ColorDialog .Color);
    ShowMessage(Format('%.2x%.2x%.2x',
                       [GetRValue(TheRGBValue),
                        GetGValue(TheRGBValue),
                        GetBValue(TheRGBValue)]));
  end;
end;