I would definitely ***not*** do stitch in the ditch! My suggestion for quilting is to use a walking foot and a straight stitch, guiding the fabric so you make softly waving lines. These can vary from each other. Your quilt pattern lends itself very well to this technique (which is also very easy to do!) because you can make the wavy lines run diagonally through your pattern. The wavy lines add a very nice "modern" touch to a quilt, in my opinion. I will see if I can find a photo:
http://www.sewmamasew.com/2014/04/qu...foot-quilting/
That is not exactly what I am talking about (wavy lines running diagonally through your quilt), but the website shows how you can achieve that effect.
I would highly recommend adding several layers of spray starch to both your top and backing fabric if you have already layered the quilt. This will help keep the fabric stable as you wander down the diagonal. If you haven't layered yet, I would spray starch the top several times and heavily starch the backing fabric (a 1:1 solution of Sta-Flo liquid laundry starch and water is good).
I do think a variegated purple thread would look lovely on this quilt.