The Series 9 #11 Green (1) Filter from Tiffen is a filter designed to build contrast in black and white photography. When using this filter for portraiture in the outdoors it will lighten skin tones and have better separation between the subject and the background. Shooting landscape photos will help darken skies and lighten and show more detail in the foliage. The filter has a filter factor of 4, so to compensate you must increase the exposure by two stops.
Tiffen filters are not dyed, but are manufactured by using ColorCore technology. This process laminates the filter material between two pieces of optical glass and grinds down and polishes the glass. The glass is then mounted to the metal ring, which after that can be screwed onto the lens for the desired effect.