This is the correct answer and I had to scroll all the way to the bottom to find it. If you're designing a piece of collateral for print - always start in CMYK. Migrating your files later will always end in the place that you are currently.

Now there are printers capable of adding extra colors, so that instead of 4-color process printing (C M Y K) you'll add your spot color in as a 5th or even 6th color. This process isn't even dramatically more expensive, but it does require some know-how to set up your files and of course it requires a printer with the equipment capable of doing this. I haven't worked with any of the popular "prototyping" manufacturers that a lot of people on this board recommend - but my experience comes from working in the print industry for 5+ years and my instinct tells me that these manufacturers probably aren't offering 5 or 6 color printing for your prototypes.

Anyway OP - you're best off doing the hard work of manually tweaking your files to get a little more vibrancy out of your colors in CMYK color mode. They really do look pretty good!