Paula of Paula's Needlework Addiction wrote a great explanation of how to change color in a pattern. She mentions using the DMC Color Card for help, when you are missing a color in your collection. I'd like to add that I find mine indispensable, and you'll want the card with floss swatches and not the one you print out.
These are fairly easy to find at various online shops for around $10. Just Google "DMC color card". Also you can check with your LNS to see if they have any in stock. You won't need the most up-to-date card with the "new" color variations, etc., just the one with the regular DMC line, so you might be able to find a really cheap one on EBay.