"Paiis" address the industry need for a rapid, simple, inexpensive method to demonstrate and assess the activities of the usually thin, invisible to the eye, photocatalytic coatings present on self-cleaning products.A "Paii", coated onto the surface of a photocatalyst material under test, works via a photoreductive mechanism, in which light absorbed by the photocatalyst drives the reduction of the dye in the "paii", thereby producing a striking color change, which can be measured through the use of a simple mobile phone camera + app, "in lieu" of any sophisticated analytical equipment.