My advice. Don't use deep blues & purples.. the most difficult for a printer to get correct.
If you don't like the colors.. let Scott know. He has worked with many people to get the colors correct. I think Scott asks for files to be in RGB mode, but you can ask him. Hey may have some sort of rip / print station that converts everything to CMYK anyway, so RGB might be the best for that. I always work in CMYK if I'm going to print to a CMYK printer. Scott's printer may have more colors or some completely different process though, so let him know