How do chromatophores allow octopuses and cuttlefishes to change colour?

In cephalopods such as octopuses and cuttlefishes, chromatophores are specialized organs that allow for rapid colour changes, often occurring in less than half a second.12

Unlike the cellular chromatophores found in vertebrates, cephalopod chromatophores consist of an elastic sac filled with pigment (such as brown, black, red, yellow, or orange-red) surrounded by a ring of radiating muscle fibres. These muscles are controlled directly by the nervous system and the brain.13

When the animal receives neural stimulation, the muscle fibres contract, stretching the pigment sac into a broad, thin disk that makes the colour visible. When the muscles relax, the sac shrinks, effectively concealing the pigment. By coordinating these expansions and contractions, cephalopods can create complex patterns for camouflage, defense, or communication, such as alarm displays or hunting signals. These physiological changes are often supplemented by reflecting cells called iridocytes, which further enhance the animal's ability to match its surroundings.1432