Cuttlefish are my favourite type of octopoid (let's stick with cephalopod then
). They use their colour changes not only for camouflage or warning but actually to communicate with their mates (whichever way you interpret that
). I think the best bit of wildlife television I ever saw was an Attenborough sequence showing cuttlefish 'talking' to each other by colour changes.
Cuttlefish or Sepias (Sepia officinalis)