I mostly like to use backings that contain colors from the top. Sometimes, it will be something really bold, other times, not so much. I only buy for projects so I will usually purchase the whole kit & kaboodle at the same time. Typically I will get binding fabric that will coordinate with both top and back.

Thread...I'm a hand quilter but the principle is the same...depends on how much I want my quilting to show - either front or back.

If I have several large scraps, I will piece my backing, buying only what I need to complete the backing. If I have no scraps, I may use something very different on the back. The binding will match/blend with the pieced top and the back, but I love to turn the quilt over and find a surprise. If I want the backing to blend with the top, I take my top with me and spend time laying top and potential backings together in the store.

I try to match the backing with the front of course. If I want wide backing Hobby Lobby now carries it, it is $11.99 a yard but with the 40% off it is reasonable and also at Hancocks using a coupon is a good deal.

I have a bolt of navy blue 108" wide fabric that is my current "go-to" fabric for backing quilts, as long as it "goes" with the quilt well enough.

I also stash 108" wide muslin, for another "generic" option. I also have several pieces of wide fabric prints I've picked up, in a variety of colors.

Bed quilts I tend to make a more boring backing for, but lap/throw quilts I figure the back is probably seen nearly as often as the front so I usually snazz it up a little with some pieced bits using leftovers from the front if I'm piecing the back out of standard-width fabric; or use a nicer printed backing if I'm using wide fabric.

I don't stress it at all! As a LAQ, I love the ease of using wide backing with no piecing involved, so I have a lot of wide back in my stash. I buy what is on sale. I have it from very busy prints to wide muslin to tonals. Sometimes I want a flannel back on the quilt. Then I just try to choose some colors or prints that compliment my top. Sometimes I use leftover blocks or scraps from the front and do a pieced back. Sometimes I don't go by color at all but go by theme, like I did a quilt with tigers and dragons quilted on the front. The quilt was black with red yellow and orange in batik fabrics on the front. The backing I chose an asian print with dragons and tigers that was a tan color so the color did not at all go with the top, aside from the dragon and tiger theme. On one recent quilt, I used an inexpensive walmart fleece blanket. In my mind its all good and there are no rules.

This may not help you for these two quilts but for the future. A lot of the time I choose my backing fabric before I make the quilt to go with it. I pull colors from my backing choice for the top. I am not the best quilter so having a patterned backing helps to hide my poor skills.

Those times I make a top before I have a backing I just take the top with me to my LQS and audition fabrics.
I love the hunt!
peace

I went to a class with a teacher and she picks a theme and uses it for ALL her backs. For example- flamingos or owls, etc. They had NOTHING to do with the front but these are her quilts so she does what pleases her. I thought it was genius.

I just try to get it to coordinate with the binding or take scraps or photos and use those to coordinate, not necessarily match.

I agree with most of the posts. If I have any extra fabric from making the quilt top, I use it up on the backing so that it does look to coordinate. But it is important to think ahead of thread and with that color will determine your backing. I was taught back in the early 90s to use a busy print on the back so any of my mistakes would NOT be as obvious as if I used muslin instead.

I buy wide muslin from Joanns when I have a 50% coupon.....muslin washes great, not a lot of shrinkage with better brands, can get up to 120" wide, matches everything, very soft after washing, and saving the best for last - it is cheap!