I have had the blue marking pin to come back after a few weeks of being washed out! I think some batches of pens may be stronger then others. If I can see a soap line I use soap slivers for dark fabrics and chalk pencils for light fabrics.

Definitely try it ahead of time. I used a yellow water soluble pencil and now hava a permanent line on a baby quilt. Don't take any chances.

I used a blue pen many years ago on a quilt piece I was hand piecing. It did not want to come out last month when I finally got around to finishing it. So now I am leery of using them.

I've been using my Prismcolor colored pencils for marking with no problem...yet.
David