That is a beautiful dress! Being that it is a summer dress, I would steer away from anything with wool. It is helpful if you can find a yarn that has a similar amount of yardage as the yarn you are looking to substitute.
This yarn is a sesame color and is on sale, if you like the color it could be a possibility:
Maybe do a search for DK weight yarns and see what you come up with. Why did you not want to use the yarn that is suggested?