Each chromatophore is a single elastic pigment sac wrapped by 15–25 radial muscle fibers, and — unlike almost every other color-changing cell in the animal kingdom — those muscles are directly wired to motor neurons, not triggered by hormones diffusing through the blood. That direct wiring is why octopus skin can change color in a fraction of a second.
The visible effect isn't a color changing — it's an area changing. Contracting the radial muscles stretches the sac flat, so its pigment covers far more of the skin's surface; relaxing lets passive elastic recoil pull it back to a tiny dot. Coverage scales with the sac's stretched radius:
Area ∝ e (e = fraction of radial muscles contracted, 0–1)
radius(e) = r_max · √e
de/dt = (target − e) / τ_expand if target > e (fast: muscle contraction)
de/dt = (target − e) / τ_relax if target ≤ e (slow: passive elastic recoil)
This asymmetry — expansion in ~150–300 ms, relaxation over several hundred ms to seconds — is exactly what the two time-constant sliders reproduce. Three pigment layers (yellow, red-orange, brown/black, surface to deep) sit stacked above a static layer of reflective iridophore/leucophore cells; because only the covered fraction of each layer is opaque, the visible mix is a simple area-weighted composite of whichever layers are currently stretched out.
- Pattern — the spatial/temporal target signal sent to the organs: a synchronized flash, a traveling band that mimics a drifting shadow ("passing cloud" display), independent random flicker, or a rapid alarm strobe.
- Drive amplitude — the peak fraction of radial muscles the pattern commands to contract.
- Time constants — push relaxation τ down toward expansion τ and the skin starts flickering symmetrically, unlike a real octopus.