mindless

I like using ultra washable markers from Crayola. Lots of colors to choose from, and, so far, all have washed out really well for me.
be careful with the tailor's chalk. some of them have wax as a base, and are very hard to wash out. those are meant to mark pieces in the seam allowances, etc. where the marks don't show in the finished garment.

testing everything is always a good idea.