Last year, I grew a bean pole variety called Trionfo Violetto, or “purple triumph.” It is a variety whose bean is solid purple and very long (some as long as a typical new #2 pencil, which is 7 inches).
The plant looks lovely with its contrasting purple and green elements, such as the purple beans against the green stalks. I thought it was such a lovely sight.
I would recommend eating them straight off the vine. Don’t pick a bunch like I did all at once because these fresh beans don’t keep very long. I’m sure there are ways to keep them longer, but really when you’re ready for a meal, pick what you have ready on the stalk.
Over 130 cultivars of edible beans are known and the Trionfo Violetto isn’t the only cultivar that has a purple pod, although it might be the only pole/climbing variety of that color. The Royal Burgundy is a bush bean that also has a purple-ish pod.