Green coffee beans are basically just unroasted coffee beans. Coffee beans are naturally green, but they are usually roasted before being sold to the consumer. This is the process that turns them brown.
As we know... coffee beans are loaded with antioxidants and pharmacologically active compounds. Two of the most important ones are Caffeine and Chlorogenic Acid.
Chlorogenic Acid is believed to be the main active ingredient in green coffee beans. That is, the substances that produces the weight loss effects.
Unfortunately.... most of the chlorogenic acid is removed when coffee is roasted.
For this reason, roasted coffee beans won't have the same effect (although there are plenty of other good reasons to drink coffee if you enjoy it).