The flavour difference between green and purple artichokes is subtle; purple artichokes are slightly heartier and earthier in flavour.
I usually soak them in water for 10 minutes, then dry them and wrap them in a cloth and put them in the lower compartment of my fridge.
How to cook
Artichokes are one of my favourites! The children like them because they "have to" eat them with their fingers. :)
Cut the stem and leave 3cm on the artichoke. Rinse the artichoke and place it on top of a steaming basket. Cover the pot. Bring to a boil and reduce heat to simmer. Let it steam for 25 to 30 minutes. Add water as needed. Cooking time depends on how large the artichokes are. The larger, the longer they take to cook. To eat them, pull off the outer leaves, one at a time. Dip the white fleshy end in a vinaigrette. This web site explains well how to do it, and it might come handy if you have never cooked an artichoke before. Enjoy! :D
Note that the weight is approximate.