The Formatt 58mm 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.