My wife just came home with two 20x30 prints from Costco and they missed the mark! The contrast was not there and the colors were off a bit. Before we uploaded the images I even called and asked them what color space they wanted them in and they said 'RGB'. I said 'be more specific, there are different RGB's out there.' The girl says 'I just know it has to be RGB'. I knew I should have viewed that at as a Red Flag. But, I sent them in as Adobe RGB.
They don't look 'bad', but they are not the same as what I see on my monitor. I have had many images printed for framing, calendars, post cards, etc, and they have always been spot on with my monitor in the past. My normal print guy wants everything in CMYK, but that was not an option for Costco.
Any idea on which color space I should have used to get a better looking print?

I passed your comments on to the team at the lab and I know they'll be delighted.
