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Tim Smith11,164 Points
Why do we use RGB? The primary colors are Red Yellow and Blue? Why use the secondary color green?
Sorry if this is off the topic of css. It is perhaps more a color theory question. :)
TJ Egan14,420 Points
At additive colors, the more you add, the brighter, because they are emiting elements. This is why the day light is (more-or-less) white, since the Sun is emitting in almost all the visible wavelength spectrum.
At the other hand, with substractive colors the more you put the darker, because they are reflecting elements. This is why the black colors get hotter quickly, because it gets all the electromagnetic waves and reflects almost none.
Specifically to your question, it depends where you are working on. Traditionally, it have been used additive colors (RGB) because the canon for computer graphics was the computer monitor, and since it's a emiting element, it makes sense to use the same structure at the graphic card (the colors are showed without conversions), but if you are used to graphic arts and press, the logic is substractive colors (CMYK) as for example Photoshop (if configured) works, so really it doesn't matter what color group you use: the primary colors of one group are the secondary colors of the second one and viceversa.
Kevin Korte28,111 Points
I'll just add that I agree that difference here is additive colors vs subtraction colors. When you're dealing with digital screens, you're dealing with additive colors. When you're dealing with print, you're dealing with subtractive colors.
It's why your computer monitor uses red, green, and blue light to create the colors on the screen, but your printer use cyan, magenta, and yellow (black too) ink to print that same website image.
Brett Cole10,181 Points
Check out this Link helps give a pretty good explanation why. (http://www.reddit.com/r/explainlikeimfive/comments/1ih41t/eli5_why_digital_colors_are_expressed_in_red/)