If your customers are printing them at home or at Staples, Office max, etc. and cutting them out set the color guides to CMYK and save the files as .PDF. If you know that they are going to print them at a photo center like wallmart, shutterfly, vistaprint, etc. then set the designs as RGB and save them as a .JPG file. But, just like PurpleBerryInk stated, other things can alter the colors such as paper types and the ink quality of printers and also- everything looks brighter on screen then it does in print. If customers are viewing your items on a smart phone of tablet the colors will really look different.
I actually state this right in my proof agreement form and at the bottom of each listing! You may want to try this also:
"*Please note that the colors you see on our samples vary from screen to screen, as well as when you print. Colors may vary slightly and can sometimes print darker or lighter than what appears on the screen, nothing too drastic though!"
It just helps to make your customers aware of this so that they know upfront and won't come back at you! :)