First things first. What are the dominant colors of the quilt top? Above, orange seems to jump out at me on first glance, followed by the blue pieces. Generally, I don't use the dominant color from the front allowing that to be the entire quilt's focal color. Instead, I look for the lighter elements. Here yellow, green, and a hint of brown are present as well as spots of cream.
The reality is- it is your quilt and how or what you place on the back of it is entirely up to you. Some quilters add an applique or even a pieced strip into their backing as a way of giving a new unique touch. Here is where you could use the left over scraps from cutting the squares. My blue version of spring picnic didn't have much excess to speak of, so that isn't an option for this back.
A few key points others have shared:
**when in doubt, yellow always pulls it together. It seems the soft hint of yellow goes well with any quilt top color pallet.
**decide your desire. Do you want it to seem almost reversible? Or a blended complement? My thought is- stick to the color pallet.
**confidence is key. How confident are you with your quilting. Patterned or figured backing fabric will forgive any error and hide awkward or uneven stitches.
**there are no rules. If you like it..use it. The quilt police will not come to your door and charge you with poor fabric choice.
Take time this week to play with the idea of backing your quilt; no need to rush. Consider the elements, the colors, even the motifs on the top and settle in good with the choice you make.