A kale plant has green or purple leaves 
I love kale! I've only ever seen the kind with green leaves, though.
Before the Middle Ages, kale was one of the most common green vegetables in Europe.
I didn't know that. I don't think of it as being associated with a lot of the stereotypical foods of Europe - say, French or Italian or British food.
Medieval cuisine
Medieval cuisine includes foods, eating habits, and cooking methods of various European cultures during the Middle Ages, which lasted from the fifth to the fifteenth century. During this period, diets and cooking changed less than they did in the early modern period that followed, when those changes helped lay the foundations for modern European cuisine.
Cereals remained the most important staple during the early Middle Ages as rice was introduced late, and the potato was only introduced in 1536, with a much later date for widespread consumption
Kale
Kale () or leaf cabbage is one of certain cultivars of cabbage ("Brassica oleracea") grown for their edible leaves, although some are used as ornamentals. Kale plants have green or purple leaves, and the central leaves do not form a head (as with headed cabbage). Kales are considered to be closer to wild cabbage than most of the many domesticated forms of "Brassica oleracea".
Etymology.
"Kale" originates from Northern Middle English "cale" (compare Scots "kail