If you're like me (ie. generally clueless), you probably don't really know the difference between RBG and CMYK coloring. Here's a concise post from DigitalWebbing by Pens&Pixels that clears things up:
The more simplistic of coloring style you use, the more reasons there are for CMYK. If you're leaning towards painted/high contrast/textured work, use RGB.
CMYK: This is how ink works. You start with white and add pigment. The advantage to CMYK is having precise control, while coloring, over the exact inks that will be printed. The disadvantage is that color to color blends are not natural the way they would occur in real life.
RGB: This is how light works. The translation to print colors may alter your exact swatches slightly, but blended color will appear more natural than in pure CMYK.
Here are a few examples of CMYK and RGB in action. The 1st Red to Blue and Green to Blue gradients were made in RGB. The bottom versions used the exact same colors in CMYK. Flat colors or very controlled blends in CMYK work great. Beyond that, RGB is the way to go.