Color with Excel and Openpyxl

Good morning, I would like to fill some cells with a gray background, the code works and the color as well, in visual code in the preview of the excel file I see the colors correctly, are that when I open the file with excel the background is always black. any help?

        def excel():
                # Writing on a EXCEL FILE
                filename = ("testexcelcolor.xlsx")
                try:
                    wb = load_workbook(filename)
                    ws = wb.worksheets[0]  # select first worksheet
                except FileNotFoundError:
                    headers_row = ['Name SGV PN', 'IVU Nummer']
                    wb = Workbook()
                    ws = wb.active
                    ws.append(headers_row)
                for cols in ws.iter_cols():
                     if cols[-1].value: # need to check the cells have values.
                            # otherwise colors the entire row.
                        cols[-1].fill = PatternFill(bgColor="FFFFFF", fill_type="solid")
                wb.save(filename)
                ws.append([ivunumber])
                wb.save(filename)
                wb.close()
        excel()