One purple tetraploid I've always liked strictly based on color is Paul Voth (Griesbach-Klehm 2008). I can't attest to it's northern hardiness since I've only grown it in NC, but I would imagine it's northern hardy since it was bred and propagated in Wisconsin.
Check out this article about Dr. Griesbach and the naming of Paul Voth.
https://magazine.uchicago.edu/...
