Hi Seth and KK,
Panther chameleons change color depending on mood, physical states, and temperature. When they need heat, they will turn dark or black in order to capture the heat from their basking lights. Black colors soak up the sun and/or the heat rays from your lighting. If your chameleon is dark in color, he probably needs some heat. They turn lighter in order to cool down and not attract the heat.
Panthers show their bright colors when they have the feeling of stress, fear, anger, mating colors, curiosity, aggression, among many, many other states of emotion. So when you take him out of his cage and he starts to turn bright colors, he is showing an emotional state. Probably either a bit of fear or stress about being handled.
When he's not really showing any color at all, an olive green or just a regular color with some barring showing (with the exception of nosy be type locales who usually are always blue), they are just "chillin." Hanging out in their cage and taking it easy.
If a panther is showing their brightest colors all the time, this is a sign that they are upset about something and under a good amount of stress. So it's a good thing that they don't always show their bright colors

.