Plutarch's Natural Questions by Plutarch

Whether, as Theophrastus writes, because it is an animal by nature timorous; and therefore, being disturbed, it changes color with its spirit, as some men do, of whom it is said, an ill man ever changes color? But though this may serve as a reason for changing its color, it will not for the imitation of colors.

For the polypus does so change its color, that it is of the color of every stone it comes nigh.