This is used to generate a matte - which is transparent wherever the blue-colour features on the original film, and opaque elsewhere.

BBC: Blue and green-screen effects pioneer Petro Vlahos dies

Each disc is covered in a transparent plastic coating (either blue, green, yellow or red) which lets daylight filter through.

WSJ: Daniel Buren's Bright Periphery | 'Excentrique(s) Travail In Situ' at the Grand Palais in Paris

Maybe it's time (once in a blue moon) for a high-level bipartisan commission--televised and transparent--to review the science, the costs and the regulatory processes, taking into account the national interest.

FORBES: A No-Lame-Duck Growth Agenda