Each pixel in your screen is made up of three sub-pixels, corresponding to the colors red, blue, and green. because of the proximity of the subpixels, rather than seeing the individual diodes, we tend to see the composite color they form when lit.
If you've ever laid ink on top of itself, it tends to get darker, no matter the color. That's how the CMYK color model works. Like RGB, CMYK mixes a few principal colors together to get a result, but instead of getting lighter as you pile on more color, CMYK is subtractive, meaning results will be darker when more color is added.