My backs are always pieced.
If I have small pieces left over from the front I make those into blocks. I put them on the floor (my design wall). Then I measure left/right to see how much fabric I have to add to create a back. Once I get the left/right done then I work on the top/bottom to see what I then need.
The fabrics are always what is on the front. I always purchase more so I can do this.