I don't think muslin comes in colors. Just bleached and unbleached.

For your backing, I would recommend simply buying some coordinating (or interesting) fabric to add strips on to get the width you need.

Are you going to have the quilt quilted by a long arm quilter? If so, you will need to allow extra inches for the backing. You should check with the LAQ to see what they require, before you purchase your backing fabric. You might have to purchase fabric and seam it to make it big enough...