All walls of buildings shall be finished with face brick, stone, modern metal paneling, wood, glass, precast concrete finished with an attractive surface or their equivalent. No unpainted or unfinished metal or galvanized metal sidings shall be permitted for any exterior walls or roofs. All building exteriors, including but not limited to precast concrete, tilt-up concrete, metal or wood partitions, shall be painted in a color approved by the Planning Board and Architectural Review Board as indicated on the required rendering of the building. Preengineered buildings are prohibited unless the building exterior is finished with face brick, stone, modern metal paneling, wood, glass, precast concrete finished with an attractive surface or their equivalent.