it depends on the quilt top-
i generally use some sort of print- or mottled fabric- sometimes i dye my own backing to get what i want- i've used panals, i've made whole new big block quilts for the back- i've used stripes, plaids, florals----what ever catches my eye & seems to me to be the perfect choice for the back of this quilt- there are no rules- it is up to you to use what you like for the quilt you are making.
one thing is ---solids will show every stitch- prints will help hide any small errors- so if you are fairly new to quilting a print of some sort will be more (forgiving) a solid will show everything.