สินค้าไม่เคยใช้กี่สี?
How many colors are never used by any product?
products_df = pd.DataFrame(columns = ['color_code']); Ref_colors_df = pd.DataFrame(columns = ['color_code'])
Ref_colors_df[~Ref_colors_df['color_code'].isin(products_df['color_code'])].shape[0]