I'm going along with Sue and PaperPrincess a dark green or deep navy blue would provide the contrast you are looking for and keeping within the Gram's color scheme. A beautiful dark green is what I personally would choose. I love the pattern and know that fabrics you currently have will be fabulous for such a design. Gram will love it and cherish you even more than she does not. Have a wonderful time creating your masterpiece, your Gram will only see the love and care you put into it for her!