I tested these markers once by drawing a square of each color on a strip of white kona cotton. I wrote the color I used beneath each color in black permanent marker so I would know which washed out and which didn't. I let it set overnight then washed the strip of fabric in the washing machine with regular detergent only. Every one of the colors washed out totally. I could not see any evidence of any of the colors.
The only thing I can't speak to is what if the marked quilt set for weeks or months before being washed. Don't know how it would wash out after a length of time.