use it for your backing fabric, add your leftovers from the front and piece the back, I do it all the time. One time I made too many 1/2 sq triangles and used them also in the pc'd back , I've used extra blocks I had, even experimental blocks using different color placements, there is no ugly fabric!
We had an Ugly Fabric challenge in our Quilt Guild this year, every one got a fat quarter of the "ugly fabric" and you'd be surprised what the gals did with it, they all came out beautiful ( we have alot of talented gals in my group) so challenge yourself ,think outside the box, I'm sure it will come out beautiful too! Jan