SueSew, what a lovely quilt! That narrow sashing adds so much!
What did you mean about purple not being allowed? It certainly worked well in your baby quilt. The 13th picture in Bonnie's intro shows the green wall, the pink couch, the black and white floor and there's a painting on the wall with blue and purple. If you cover up the floor, you can imagine how the palette would look with purple instead of black.
susiequilt, I would avoid using a thread dark enough to be conspicuous with the whites, so will probably use my go-to neutral, which I would call a warm beige.