OK.... several suggestions, GEMM
1. Do you have any of the hydrangea fabric left? Using small scraps make a single strip the width of the quilt and add a complimentary fabric above and below to piece a back.(I do this when I have a back I like when it is too small.)
2. Give yourself the grace to purchase just enough fabric to finish the back with a fabric that pleases you. We are all endeavoring to shop from our stash FIRST but there is no rule other than the ones we make for ourselves that says we can't shop for what we need to beautifully finish our quilt project.
3. If scrapping together a bunch of coordinating colors from the front and use that for the back doesn't make you happy, you will never be happy with the end result even if it IS all from your stash. It is your quilt. Make it something you can be proud of.
4. Nothing wrong doing what you suggested. Box it up with the remaining scraps and the pattern and put it away for a while. You may find something unexpected in your stash, find something perfect and drastically on sale, or on a give-away table or from a quilting friend.
Be gentle with yourself.
PS. I often work the design process backwards. Some of my best stash busting quilts started with the backing fabric and then I proceed to match colors from the backing for the quilt top that were in my stash. Sadly my backing stash has dwindled to near nothing now. But that is a good thing. Right?