3 useful colours: white, blue, and bright red. Use them for whatever colour combination you are working with.
Simply draw as you would with a chalk pencil. Wipe away with a damp cloth or sponge OR simply let the fabric sit in the open and the markings will evaporate within 48 hours (time is affected by humidity). Yes, the markings will disappear completely without a trace but try the pens on a small hidden spot first, if you are worried.
Unlike chalk, these pens do not break or crumble or leave powdery residue on your hands, fabrics or your work space. Inhaling the powdery residue of chalk can pose problems for people with respiratory issues or with prolonged exposure. No such worried here but please, do not give to small children to avoid misuse, such as swallowing or inhaling the caps.