Thanks everyone!
I have used these markers under extreme conditions! I have used them with starch, hairspray, I've ironed them, I've gone over them in different colours and I have never had any issues with the colours coming out. Any of the colours.
You do, however, have to wash the quilt on a full, heavy duty load. I washed one once on a quick cycle, and some of the colours didn't come out. I almost panicked! But I put it back in the machine on a heavy duty load, and there were no more markings left. I don't use anything special to wash / treat it before it goes in the machine. I use unscented no name laundry soap and that's it.