Subtractive Colour Mixing using cyan, magenta, and yellow. This simulation shows three light filters used in subtractive colour mixing. Each filter allows through two primary colours and subtracts (filters the other one):
CYAN subtracts RED passes BLUE & GREEN
MAGENTA subtracts GREEN passes RED & BLUE
YELLOW subtracts BLUE passes RED & GREEN
CYAN + MAGENTA yields BLUE (cyan removes the red and magenta removes the green)
RGB -R -G = B
CYAN + YELLOW yields GREEN (cyan removes the red and yellow removes the blue)
RGB -R -B = G
YELLOW + MAGENTA yields RED (yellow removes the blue and magenta removes the green)
RGB -B -G = R
CYAN + YELLOW + MAGENTA yields BLACK (cyan removes red, yellow removes the blue, and magenta removes the green)
RGB -R -B -G = 0
Subtractive colour mixing occurs with pigments rather than lights (lights use additive colour mixing). Printing uses this technique. Printing inks use CYAN, MAGENTA, YELLOW & BLACK (CMYK), the K referring to the black ink. Black gives a richer contrast and uses less ink.