Sepia officinalis has one type of photoreceptor pigment in its retina. By the ordinary definition, that makes it monochromatic: it cannot compare signals from two pigments to extract hue. Yet placed on a coloured substrate it generates patterns that fool visually hunting fish.
Part of the resolution is that camouflage rests more on pattern, contrast and texture than on hue. Cuttlefish skin control is driven by chromatophores under direct neural control, layered over reflective iridophores and leucophores that passively scatter ambient light. A white leucophore takes on whatever colour surrounds it without the animal needing to know what that colour is.
A further proposal, from Alexander Stubbs and Christopher Stubbs, is that cuttlefish exploit chromatic aberration: their W-shaped pupil and off-axis lens spread wavelengths to different focal depths, so accommodating the lens could recover spectral information. That hypothesis is not confirmed, and how much colour information cuttlefish actually use remains an open question.