Hi Lea,
The quilt looks fabulous! I love it!
I also have had the bad experience of chalk ...I learned something. You can use colored chalk on any fabric and it washes out...however if you use white thread it will permenently dye the white thread. Because of the process of thread they bleach all white thread ( all brands) it makes that thread very porous and readily able to soak up the colored chalk. So it doesn't come out. I learned the hard way myself.