A cuttlefish is almost certainly colourblind, and still out-camouflages every animal in the sea. The skin is the trick. Sepia officinalis carries up to around 200 chromatophores per square millimetre, each an elastic sac of pigment ringed by radial muscles wired directly to the brain. No hormones, no waiting. The muscles pull, the sac flattens into a disc, and the colour appears in tens of milliseconds. Relax, and it snaps shut by elastic recoil. A display panel driven like a muscle. Underneath sit two more layers doing the optical heavy lifting: - Iridophores, stacks of reflectin protein platelets acting as tunable Bragg reflectors, producing structural blues and greens no pigment in the animal can make - Leucophores, broadband scatterers that bounce back whatever ambient light is present, so the animal matches local illumination without measuring it The eye is the strange part. One visual pigment means one channel, which should mean no colour vision. But the pupil is a W, the len...