Differential output of motor units during neuro-muscularly generated colour change has been assessed by comparing the expanded sizes of elements served by units that darken the skin of a squid, with their size at rest in a pale skin. The wide range of resting sizes of pigment spots (retracted into a tight sphere at rest, Fig. 1 left) is thought to correlate directly with ontogenetic age of chromatophore organ [1] – thence with the age of motor neurons supplying them. Maximum ratios of expansion (areas) subtended by individual brown (B) and red (R) spots when the skin is dark (cf. Fig. 1 right), range from c. 30:1 to c.20:1; converted to mean diameter, ratios at any moment are a measure of the shortening of chromatophore muscle and thus of firing frequency [2, 3]. Figure 2 shows the contribution that red/brown spots make to one of the common camouflage dresses worn by a squid. At medium intensity of expression, relative power falls off with increasing resting-size/age of chromatophore organ. [N.B. were the squid to have turned all-dark (maximum intensity) the curve would be nearly flat]. Evidently fine gradations of tone are achieved across a limited range of firing frequencies (data not shown) by bringing in the units of small (younger) spots first and of large (older) spots last. It recalls the way motor-unit recruitment grades the power output of vertebrate limb muscles – with here an additional developmental signature.