I used to match my threads with the majority of the fabric in the quilt but I tried red thread on black and fell in love with contrasting threads from then on. Of course it doesn't always work out for every quilt but sure makes a statement as your sample pieces show.
I'm helping a lady with her 1st quilt done in scrappy pinks and yellows with unbleached muslin for the sashing. I pulled out my threads for her to pick for the quilting and she came up with a reddish purple, one of Connecting Threads newest colors called Sangria. I laid it up against her blocks and I liked it. The backing will be made from the leftover unbleached muslin so will be stunning too. Of course I can't quilt like you do plus I use a robotic system for mine as my hands tremor way too much but I sure enjoy your talent.