a chameleon's color changes based on a change in their body temperature to adjust to a warmer or colder climate.
When a chameleon is warm they will turn bright green in attempts to cool off to a temperature more comfortable to them.
It means that it could be angry or stressed out, much like when dark green, this indicates their condition and their mood.
Male chameleons tend to be a brighter while females may be a more dull color with different shades of green.
When needing/wanting to change color these crystals move around either rapidly or slowly. This changes the wavelengths of light, thus creating the color that is seen.