If someone is offering something for sale called a yam, be sure it is Dioscorea
batatas. There are some varieties of Ipomoea batatas, the sweet potato, which
are sold as yams. Both species are edible, but they probably taste different.
Your so-called purple yam is probably a sweet potato. Be assured it is edible.
The purple color is simply anthocyanin, like the color in purple cabbage. It
will probably dissolve in the water or turn color if you cook it. In this
country there is a big, sweet, orange-fleshed sweet potato which is often sold
as a yam.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"When you come to a fork in the road, take it." Yogi Berra