The fava bean, often referred to as an "Old World" bean, is one of the oldest domesticated food legumes. It is thought that the beanstalk in "Jack and the Beanstalk" was probably a fava plant. I recently received a question about green verses brown fava beans. Vicia faba come in a variety of shapes, sizes, and colors. You'll find them in brown, light tan, green, and even purple. The favas that are usually used on St. Joseph Altars are dark brown or beige. This is the natural color of the dried fava.
The University of California Small Farm Program has a very information page on favas. And, here's info on the fava and its place on the St. Joseph Altar.
Here are some images of different colored favas.