Those are beautiful!
The only thing I would suggest, is that she paint the design in front of the fletching.
It is being hidden by the fletching. Very nice indeed. Basically marbling is a blend of colors, that are not stirred to a solid blend. You still have the various colors, and one or two slow stirs, and then you either paint, or dip the object into it. Or I think you can spray on one color, and then another, and gently wipe it around, or drag one into the other, and roll the shaft,and it will semi blend, with each color showing, and some solid blends as well.
Wayne