Don't worry about it! That little bit of brown not being washed will not matter at all in the grand scheme of things. Just quilt it and finish it. Once the fabric is sewn into the quilt, all the other fabric, batting and quilting stitches will control shrinkage. It will be one unit then.
Honestly, it will not make a difference!
Don't you hate those "OH NO" thoughts you get lying in bed?!
Very pretty quilt, BTW...