I, too, think you're doing a great job on this quilt! And I like the colors. If you know the finished size of your quilt and you see something you like, go for it. I try to pick backings that compliment the top. That is, have some of the same colors and similar scale - usually. Sometimes there is something totally different that just screams 'me, me, me' for a given quilt. You'll know it when you see it. Just make sure you over-buy a bit so you have a bit of wiggle room if you need to piece the backing to get it to the size you need.