Everything has both yin and yang aspects as light cannot exist without darkness and vice-versa, but either of these aspects may manifest more strongly in particular objects, and may popebb or flow over time.
The colored dot inside each opposite force represents a presence of the other element. In a seemingly evil person, there also exists good. Where there is cold, there also exists warmth. Where one opposite seems to rule, there will also be a part of the other opposite within it somewhere.