I'm assuming you want to maintain the look and style of the quilt tops? If so, muslin would work on any of the 3 for backing, especially the appliquéd cats one. While I appreciate the charm of tied quilts I've only done one, but if it were me I would definitely use red on the Sunbonnet Sue quilt, at each cornerstone and incorporate it into the bonnet tie area on the Sb Sue figures. I'm sure others will have wonderful ideas for you. It's great of you to complete these. Are they a part of your family heritage? If so, don't forget a label with your family's information. Can't wait to see how you finish them!