I have used all kinds for the backing, white on white,off white on offwhite,matching a color in the front,wide backing fabric,a totally different color all together from the front, sometimes same color as the front sometimes whatever it takes to cover the back. Not a big deal to me. I've never entered a quilt show and never plan to. Once I made sis-in-law a tied comforter with a print, medium green, tiny blue flowers,and white daiseys,yellow flowers, backing matched the green on the front, she told me she liked the green side the best. I could have passed out. I thought the print on the front was so bright and happy looking. My Daughter told her the green was the back. She was sorry.
Use to be, back years ago muslin was all that was used for the backing, usually unbleached.