What is the shell of an orange called?

Zest is the outer, colorful skin of citrus fruit and is often used to add strong flavor to foods.

https://en.wikipedia.org/wiki/Image:Zest_Orange.jpg

rind n.

A tough outer covering such as bark, the skin of some fruits, or the coating on cheese or bacon.

http://dictionary.reference.com/browse/rind

zest n.

1. Flavor or interest; piquancy.
2. The outermost part of the rind of an orange, lemon, or other citrus fruit, used as flavoring.
2. Spirited enjoyment; gusto: “At 53 he retains all the heady zest of adolescence” (Kenneth Tynan).

http://dictionary.reference.com/browse/zest

peel n.

1. The skin or rind of certain fruits and
vegetables.

http://dictionary.reference.com/browse/peel