A green husk is the husk of a young coconut fruit. A brown husk is the husk of a matured coconut fruit.
Many coconuts are harvested for human consumption. In these instances the fruit is still young and the husk is still green. Young, or green husk, although a lot easier to process, is however not ideal for the production of growing mediums. The main reason for this is that once the coconut is opened and the husk removed, the husk will naturally start decomposing. In green husk the initial decomposition is quite rapid. During this phase the green husk will use all, or most of the available nitrogen. If green husks were therefore used as a growing medium, the growing medium as well as the plant will be competing for the available nitrogen.
Brown husk on the other hand is matured hard coconut husks. It is woody in structure and a far more stable substance.
Progrow cocopeat is only made from matured brown coconut husks.