I tested the Birch blue water erasable marker today. I am REALLY happy with it. It leaves a nice clean line and disappears practically the second it touches the water. I tested it on white 100% cotton with some printing on it to see if it would stain the print - not a sign of it. Brilliant!
It's a very light blue though. For this project that's fine because I'm working on white, but I'm not sure how well it would show up on other colours.