|O.k., here's a tip I got from my friend Saskia.
Use sharpend soap bars ( the tiny bit, that is always left and too small to wash your hands with ) to mark your fabric, so you can be sure, that your markings will wash away easily. A great way to mark delicate fabrics too.
It doesn't matter what colour your soap has, it will always leave a white line and the soap can be sharpend just by "washing" your hands with it.
Hotelrooms often offer these little soap bars. They are great for this purpose, because they are small and so you ave reduced it to the propper thickness soon.