I decided to stitch all my rainbows in royal/navy blue thread and to blanket stitch each one in a different thread. That way when the quilt is done I'll have a sample I can look at of some of my threads and their varying weights.
The pink center is stitched in Wonderfil 40 weight rayon. The red/orange center is stitched in Fufu polyester embroidery thread in 40 weight. The turquoise center is stitched in Glide poly 40 weight thread. They all work lovely.
What I love about using a 40 weight thread to do double blanket stitch is the thickness you get. While I was stitching these in the contrasting dark blue you could really see every error I made in stitch direction, which is good to know, especially for beginners....math the fabric or vary it slightly as you're learning.