Personally, I would go with something closer to the light color in the quilt. If you wanted to go with flannel, could you use a lightweight/thinner batting?

I like for my backing to be a surprise, something that you would not expect when looking at the top. I would use some type of a small print in a color you would not expect. Whether or not to use cotton or flannel depends on how heavy a quilt you want and like. You probably have a larger choice of material in cotton. The quilt is beautiful.

I like the way you did the quilt

I didn't notice if anyone mentioned the WideBacks site. When I started reading thought your question was about color. My advice is pick a color to match or blend with whatever color you want to quilt it in. Hides any boo boos.
I'm expecting a customer to bring me crown royal bags for a quilt he wants. I love your choice of design.
Happy Stitching

I vote for cotton.

Gorgeous quilt!!!!

Gee, that's 2 in one week that I have seen. Special to me as I plan to make one for my brother. All the suggestions re backing is helpful to me too. Never knew other people also saw " a quilt" with the bags. I would like to find a way to use the whole front of the bag with the gold stitiching. Have too many things on go right now to do a design anyway. good luck with backing.

Great quilt.

Joanne's.com and Nancy's Notions have nice wide backings at a reasonable price. I would also go with a cotton in blue and white print.

Beautiful top! No suggestions about the back except a not too tiny purple and white print.

Lovely quilt top! I'd look at some prints that have varying shades of purples and maybe small amounts of white -- depending on your taste, that could be florals, a nice multi woven plaid, multi colored circles, or even an off white plain....either cotton or flannel would be good, depending on how you intend to use the quilt after its done.