Smoke. There shall be no emission at any point, from any chimney or otherwise, of visible grey smoke of a shade darker than No. 1 on the Power's Micro-Ringelmann Chart, McGraw Hill Publishing Company, 1954, except that visible grey smoke of a shade not darker than No. 2 on said chart may be permitted for not more than four minutes in any 30 minutes. These provisions, applicable to visible grey smoke, shall also apply to visible smoke of a different color but with an equivalent apparent density.