I prefer the purple water soluble pens. You must NOT iron them or the color becomes permanent. I have left them on a quilt for a long, long time & the ink went away with no problem, but I avoid leaving them for very long periods because it makes me nervous! There is a white pen that is water soluble that works nicely on dark colors. I haven't used it much so I'm not as sure about that.
I worry about what the gel pens may do to my fabrics long term. They do not wash out. I know not every one likes the purple pens but I've had good luck with them.
Jo