Whenever the crystalline of the eye is colored as a result of a certain affection, it seems to us that the air and the other visible things have been colored with the same color. [...] He says that the visible, what by itself exists in the domain of the visible, is color, and says that surface is what is visible by itself.[1] Surface is visible by itself, although not such as [something] is by itself on account of definition. For [the expression] 'by itself' is double [in its meaning]: either it is that which is taken in the definition of the subject, such as 'animal' and 'rational' in the definition of man (for these belong by themselves to man), or that whose subject is taken in the definition, such as number in the definition of odd and even,[2] and nose in the definition of snubness.[3] For these belong by themselves to the definition. Now, it is not thus that he says that surface is visible by itself, but insofar as it contains in itself the cause of being visible. And this is color, for the color existing in surface is what is visible, and this is what the eyes perceive [...] the color existing in volume. For in the case of transparent stones, even though we think we see them colored through the whole volume, we are deceived with regard to our eyes. For the eyes or [rather] the activities of colors, because the stones are transparent, pass through their depth. And we perceive the stones through the color existing in their surface. This is why we also think we have seen the color existing in the depth.
The Pisidian [writes]: "these things are not decorated for the sake of gratification; all the arguments of truth are simple."[4]
On color, search in the [entry] 'gray'.[5]