Cardamom pods come in three colors: black, green and white. It turns out that each color has a distinct personality, flavor-wise. Green cardamom is the one you'll generally see in grocery stores and many spice shops and it turns out that white cardamom is simply a bleached version of that. Apparently the thought behind this is that you can still impart the subtle cardamom flavor into baked goods or cream-based desserts without adding any color. And according to Cook's Illustrated, black cardamom isn't cardamom at all but a close relative.
When it comes down to it, flavor is the important consideration, so the team at Cook's Illustrated did a taste test using the spice in a simple sugar cookie and in the cooking water for chicken biryani. The result? Green cardamom ground from the pod was the "most vibrant and balanced" whereas the white cardamom was a significant disappointment. The black cardamom tasted more smoky, so it'd be nice in more savory dishes but in the cookies it came across as a bit schizophrenic. The verdict from Cook's Illustrated? Go green.
Related: From the Spice Cupboard: Cardamom