I would also use black thread.
Perhaps down the middle of each dark strip .
I have found, sometimes stitching in the ditch can be a little challenging, as I jump out a lot.....
Does you walking foot have a guide...if so just find the center of your strip, set your guide and off you go....remember to take your time, no need to sew super fast...in this case you want to be the turtle not the rabbit....take some breaks often, walk away...they go back. do NOT expect to finish in one day....tell yourself its going to take a week....and then when you finish in a couple days you are ahead of the game.....Just take your time, your quilt is beautiful, you have invested a lot of time and effort in it, the quilting is the final touch on your quilt and brings it all together.