For tradition (red) Rooibos, the green leaves and stems of the plant are crushed and "fermented" before drying.  The fermentation step is actually an oxidation process brought about by enzymes and chemicals naturally present in the plant.  In the case of green Rooibos, the fermentation process is skipped, and the "green" leaves and stems are dried directly.  Different process are used to prevent oxidation.
Source: Rooibos Council of South Africa Website