Pigments not only help make things seen, they help things see. Pigments in the eyes of creatures are crucial players in the transduction of light. The absorption of light is essential for sight and thus a perfectly transparent creature would be utterly incapable of seeing anything. (Ditto for Cartesian souls).
If eyes have to be visible, I wonder what generalizations we might make about other sensory organs. I recall hearing that ears make noise in a way that helps hearing, though I don’t recall the reference or whether this is a claim about an essential property of audition.
Tongues have flavor (which is why they’re at the butcher shop), but must they? Is their taste essential to their being tasters? Noses smell in at least one sense of the word “smell”, but must they smell in the sense of having an odor? I suppose that you couldn’t make chemoreceptors out of totally inert elements and thus, the ingredients of chemoreceptors, being reactive, must be detectable by at least some other chemoreceptors.