That looks like purple striping that appears in some varieties. "A stringless mutation out of the well-known variety Triomphe de Farcy. The pods are 15-16 cm long and have a medium green color with purple stripes who disappear after cooking. In a young stage the pods can be picked as haricot verts with a length is 13 cm and a width of 6 mm. Resistant to Bean Common Mosaic Virus."