All depends on how you need to print this and if it runs by itself or is printed in conjunction with other CMYK images on a printing form. If it runs by itself you could replace the Cyan channel with a Pantone blue that has a little PMS Green as part of its base color (perhaps PMS 312 or somewhere around there). It would modify the color tones within the silver star somewhat (but not dramatically) and would get you closer to the blue background you need. The other option is to add another channel to print a 5th color (PMS your blue of choice) as the background. This would add another printing plate & wash-up to your printing but wouldn't effect any other CMYK images on the printing form and wouldn't effect the silver star either. Extra cost to do this shouldn't cost more then around $150.00 - $200.00 (depending on quantity). As noted by Hoofdcommissaris, CMYK color space is much more narrow then RGB. If you plan to print in CMYK you should almost always begin with CMYK, especially if you need to run your designs by a client.