The label on all paints, as defined in section two of this act,¹ which have been extended or reduced or compounded by the addition of inert extenders or other similar paint materials, shall be labeled with the word “compound” immediately following the title in conspicuous type: Provided, That where the percentage of oxide of zinc, red lead, white lead, natural colors, or natural colors ground in oil paste or semi-paste paint, is fifty per centum (50%) or more of the compound, the terms oxide of zinc, red lead, white lead, or the name of the natural color or natural color in oil may be retained on the label with the word “compound.” In the case of natural or chemical colors, where the inert material or other foreign substance is a natural or normal part of the product itself, the word “compound” need not be added to the name of the title.