RGB vs CMYK…a short essay on colors!
Look very, very closely at your computer screen, and you will see tiny dots of color where you thought you were seeing white. On a monitor, Red, Green, and Blue lights make White. Take away them all (no lights) and you get Black.
In the printing world, color works just the opposite. Mix Cyan, Magenta, and Yellow and you get Black, or close to it. Take them all away and you ‘re left with a plain white paper. (most printers have an extra color, Black, or Keytone, to help make images really pop!)
The difference between these two color systems is the way light is used. On printed paper, light is reflected. A light beam falls on the paper, the paper soaks up some of the light and bounces back the rest, what you actually see. However, on a computer screen, the light beams directly from your monitor and goes straight to your eyes.
Each system has colors it’s better at displaying. Depending on which mediums you use to display your logo, you should have your logo file saved using the appropriate color system to get the most accurate color. If you use your logo primarily on the web, make sure you use the RGB color system. If you just want business cards to hand out, use CMYK settings. Or just get one of each. (You CAN have your CMYK and RGB it too!) 
*To be fair, there are other color systems besides RGB and CMYK, but these are the two most common. Fancy cameras and photo manipulating programs often have their own special color systems.
Tags: CMYK, color theory, light, RGB