They wash out easily and, if you make occasional mistakes in marking, can be erased with a soft fabric eraser. One or the other will show up well on almost any fabric color. The second type is chemical markers. These are usually pens which make a blue or purple mark and are supposed to fade out automatically after a number of hours. Some do, some don't, and some fade partially leaving faint brown stains which remain forever.
Results vary with both pen and fabric.
If you use them be very sure to check first on a fabric scrap. It makes a thin purple line which fades in 24-36 hours and which is removable with a damp cloth or sponge to correct marking errors. It can be quite useful on print fabrics. The rest of us need some help. As with other supplies, a variety of measuring tools can be purchased.