Sorry about the soft focus on the photo. I spent some time playing with Cretacolor aquastics the other day. These oil based water soluble crayons color beautifully. They are creamy smooth and vibrant, the color literally glowed on the dark blue fabric. The interesting thing about these is that you can color one color solidly on top of another and then scratch away the surface to reveal the color beneath. This technique is called sgraffito. The downside to these crayons- they are NOT fixable. I tried letting them sit for 24 hours and heat setting them with an iron and they were still water soluble. So they are probably not well suited to fiber art because of the delicacy of the surface. I think these are best to use only on paper.
Next I tried the Neocolor crayons, these were as vibrant, and smooth as the Cretacolors but they are not water soluble. They are a wax based crayon, that held up well to the iron. On white fabric I noticed no difference in the color vibrancy. On dark blue it did loose a little of the brightness as the wax melted into the dark fabric somewhat, but it still looked good. So I think I would try these for possible work on fabric, although I have not sewn on it yet, I love the way it looks.
This was another experiment to see if the Neocolors might act as a resist, like batik, by drawing on the fabric and painting over it. There was a little bleeding but it doesn't bother me too much, I like the playful quality of it. So I think it is a good medium to use with paint.