What is the difference? RGB stands for Red Green and Blue, CMYK stands for Cyan, Magenta, Yellow, and Key (Black). Here is the skinny on both of them.
RGB makes all colors with a mix of those three (red, green, blue). It is an additive color model, which really means if you add all of them together you get white, not black. This is because it is the color mode of items that emit color, not reflect it, i.e. screens. That is why RGB is the preferred color mode for digital design.
CMYK does much the same, but with pigment. You see the colors because the pigment is not emitting light but reflecting it. It is a subtractive color model, which means when you mix all the colors they get darker, not lighter.
Because of those factors, CMYK is the preferred color set up for print. Both offset and digital printers use CMYK. Offset printers use screens, one for each color, that determines where the pigment goes on the paper. The K for Key may sound strange for black, but it serves a dual purpose.
The obvious one is that if it was CMYB some people would think the B might stand for Blue. The other reason has to do with the printing method; press techs would use the black color plate to line up the other colors since the black plate was usually the easiest to see the outlines on, therefore that plate was “Key.”
If you have ever uploaded an image to a website only to be surprised that the reds suddenly were glowing like a Christmas light and the blues were now a bright purple, you probably had the color mode set to CMYK when you saved it. Web browsers aren’t very good at decoding color, and end up interpreting them in wild, unpredictable ways. Same can be said for printing pictures that are set up as RGB.
RGB can display more colors than CMYK, even though there are only three colors, the value associated with each color goes from 0 up to 255. That is a total of over sixteen and a half million colors. That said, CMYK will always print better. There are limitations to what CMYK can produce, and you should prepare clients for those limitations; four colors can only produce so many variations and will fall short when you hit the neon end of the spectrum. I find that a little explanation and education set most clients at ease.