(Info and image from Island's Apart included)
Many people have been saying Cassandra's skin tone has become paler due to her utilizing the moonstone's powers. As much as I agree with this, her skin may be pale due to lack of sunlight as well.
Now, the moonstone is life draining, BUT we've seen Cassandra using it during nighttime. We haven't seen her use it in daylight yet, although we may see her use its powers during the daytime in an episode soon.
According to studies, a lack of Vitamin D/sunlight causes paleness in most humans, and therefore this may be the reason why Cass's skin maintains a lighter tone.
(Or its just the moonstone stealing her life...)