private void setupColorMap() {
        if (wb instanceof HSSFWorkbook) {
            helper = new HSSFHtmlHelper((HSSFWorkbook) wb);
        } else if (wb instanceof XSSFWorkbook) {
            helper = new XSSFHtmlHelper((XSSFWorkbook) wb);
        } else {
            throw new IllegalArgumentException(
                    "unknown workbook type: " + wb.getClass().getSimpleName());
        }
    }