Years ago, I used to plant a variety of French green bean (haricots verts) called Régalfin, which was available from Le Jardin du Gourmet in Vermont. The bean pod was light green with purple flecks, and when it was cooked up, the entire pod turned a deep green. Régalfin was a prodigious producer of very tasty beans, and although it was not stringless, as long as the beans were picked young, the strings weren't a problem. The variety no longer seems to be available.
After several years of not planting beans, I decided to bring them back to the garden, this year planting Velour, a variety I purchased from Jung Seed in Wisconsin. (Actually, I purchased the seeds last year but never got around to putting them in the garden.)
Velour is a purple variety, which presumably will turn bright green when cooked. I just harvested my first batch of beans, and we're having them tonight at dinner. As you can see in this photo, the beans are indeed purple, though some of the pods that were shaded by the leaves are a paler, mottled green-and-purple color.
Jung promises that Velour is strong producer. We shall see. The critter in the yard so far has not bothered the beans — he is already eating green tomatoes — and if the critter's attention turns to the beans, that will undoubtedly reduce yield. But the plants so far seem to be very productive.
But the proof of the pudding, as they say, will come tonight. I'll let readers know the result.