<p>Why do computers use RGB (red, <em>green</em>, and blue) values for color composition rather than the primary hues, red, <em>yellow</em>, and blue?</p>

The hues of magenta, yellow, and cyan are primary for subtractive combination (e.g. paints or inks) rather than additive combination such as light where red, green, and blue are primary.
Wikipedia has more detail on the whys and wherefores
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