Black ...
Though I would audition the black, along with the purple ... black the width of the 1/2 hexie, then purple, then more black, probably the width of the first black plus the purple. Possibly wider. Then bind it off with the purple. Yes, I am thinking leave off the striped border. For a reference of the original fabric, include it on your backing.
If you decide on the purple as I suggested, .... don't introduce a new fabric. Just cut the purple stripe (with seam width) from your original fabric for a perfect coordinated effort. Likewise use the original fabric for the binding.