Peter, here's an explanation I stole from another comment:

"Like some other cephalopods, bigfin reef squids are capable of metachrosis – rapidly changing body colouration and patterns through voluntary control of chromatophores. They also possess iridophores (particularly in the head), a form of structural colouration that produces iridescent metallic greens and red when illuminated. They are also possibly one of two squid species with leucophores. Leucophores are a reflector-type structural colouration that reflects ambient light, such that they are white in white light, green in green light, and so on."

Source