The Cokin Z-PRO 022 80C Color Conversion Filter is blue. It is used to balance daylight film under artificial, tungsten or standard (household) incandescent lighting.
What happens when you want to take a photo of your grandmother in her apartment and she's sitting next to an antique table illuminated by an old antique lamp? You'd like to capture the essence of her age but don't want to make her look too pasty. Your daylight film with an 80C filter will leave just enough yellow in the image to give Grandma a flattering warmth.
This filter allows you to somewhat "balance", i.e. bring back, correct color tone to your images. There is some loss of film speed that may require the use of a tripod but, at least, your photos will demonstrate the proper tonality they so richly deserve.
The Z-PRO Series is recommended for focal lengths of 20mm onwards (in the 35mm format). This filter is made of CR39 Resin which is a reputable lightweight and shock-resistant optical material also used for ophthalmic glasses.