complementary colours

Definition: Pairs of colours which produce white when added together as lights e.g. secondary colours -- cyan, magenta and yellow -- are complementary to primary colours: respectively red, green and blue.
Compliment them on their colour
Coloured filters generally transmit the colours of its colour-name while absorbing light of the complementary colour. Given our fickle language skills, the most common filter -- the UV filter -- breaks this rule: it stops ultra-violet while passing all other visible wavelengths; it should be called a minus-UV. A true UV filter appears black.

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