All print files should be in CMYK mode as this is the mode required for the printing process if you want the most predictable outcome. If we receive files in RGB mode, we'll need to convert them to CMYK, which may shift the colors some. That said, if you are looking for a brighter color outside the typical CMYK color spectrum we can often obtain brighter blues and reds using RGB. Fill free to talk to your project coordinator about this.