|I once had buttonholes professionnaly done on a winter wool coat I made. I was surprised to see the man using an old bar of soap instead of the usual chalk to mark the button holes. I went home and tried it and have been using soap since then to mark fabrics.
The idea is to use a bar of soap that has gotten very thin let it dry and just use it as is. I must say it stays more sharp than the chalk , it does not break, does not stain fabric, lasts fortever and brushes of well (although I recommend to do some tests).
It works great for me I have been using that on medium to dark color fabrics.