Frixion Colorsticks erasable pens by Pilot. These colorful gel ink pens write on fabric, and erase completely with a hot iron. Use them for embroidery, cross stitch, sashiko and more!

From the manufacturer: ColorSticks delivers an ultra-smooth, erasable writing experience with a lightweight, streamlined, capped, clipless design.
Writes smooth & erases clean, so you can write, erase & rewrite repeatedly without damaging documents - no wear or tear!