Why do computers use RGB (red, green, and blue) values for color composition rather than the primary hues, red, yellow, and blue?
RGB for color composition rather than primary hues
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Related Solutions
This should work - just linearly scale the red and green values. Assuming your max red/green/blue value is 255, and n is in range 0 .. 100
R = (255 * n) / 100
G = (255 * (100 - n)) / 100
B = 0
(Amended for integer maths, tip of the hat to Ferrucio)
Another way to do would be to use a HSV colour model, and cycle the hue from 0 degrees (red) to 120 degrees (green) with whatever saturation and value suited you. This should give a more pleasing gradient.
Here's a demonstration of each technique - top gradient uses RGB, bottom uses HSV:
The method could vary depending on your needs. Here are 3 ways to calculate Luminance:
Luminance (standard for certain colour spaces):
(0.2126*R + 0.7152*G + 0.0722*B)source
Luminance (perceived option 1):
(0.299*R + 0.587*G + 0.114*B)source
Luminance (perceived option 2, slower to calculate):
sqrt( 0.241*R^2 + 0.691*G^2 + 0.068*B^2 )sqrt( 0.299*R^2 + 0.587*G^2 + 0.114*B^2 )(thanks to @MatthewHerbst) source
[Edit: added examples using named css colors sorted with each method.]
Best Answer
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.