Try a varigated thread. I use these a lot and like the effect.

If I want to do straight line quilting, very seldom, I line up my walking foot with the seam and stitch about a 1/4" from the seam, works better that way for me. Or I run stitches corner to corner and just eyeball it. I usually only do FMQ, large stipple, or meander. Washing and drying hides a multitude of mistakes.
Good luck.