** Caveat – I am not a graphics person.  What I say is from a game designer / Disaster Recovery / Computer Networking / Satellite Networking perspective ***

CMYK stands for Cyan, Magenta, Yellow, and Key (Black).  So what?  RGB stands for Red, Green, Blue…. again…. so what?  Here is the very simple explain:  

RGB: These are color standards that your typical graphics applications adhere to.  RGB is for computer graphics.  It’s meant for a computer screen or maybe a movie screen.  Backlit and using display technology to generate the image.

CMYK: if you are a board game designer – this is your graphics standard.  These are built for printed display.  On paper, with ink.  Not backlit.  So if you compare – RGB will have MORE colors, and they will be MORE VIVID.  If you design something that will be in printed format using RGB it has a high probability to end up looking dull, weird and likely terrible.

So if you are not doing things in CMYK – you are likely doing it wrong or have no regard for the color of the final product.  Free and cheap tools for RGB abound (computers are RGB friendly, with their displays and all) and this may be fine to prototype – but be aware that the colors and appearance will not be what you see in print.  CMYK is just the beginning – to go hardcore you need to calibrate your display to the printers, and even more… in a past life I did this for customers in the paper publishing industry.  I’m going to gloss over all of the technical details and keep it simple though.  Stick with CMYK files.  The rest can be left to your professional graphics person.