I read it as saying they're conventional in the sense that the lines between categories of sensory experience are drawn by consensus, lacking direct access to the experiences of others.

We of course lack direct access to the atomic-scale world as well, but I imagine that's the point -- atomism was a lot more abstract to the likes of Democritus than atomic theory is to us. The underlying physical reality is in a certain sense abstracted away from us, and we reflect that by talking about physical experiences in a conventional way, but those experiences are still rooted in the reality of atoms and space -- or distributions of probability density, if you prefer.

Including three kinds of light sensors, including one for around 700 nm electromagnetic waves (also called "red"), is a common design convention for humans (for reference, see "Further Random Experiments with Photoreceptors in Furry Legged Things", in Proceedings of the Council of Azathoth, 63 million y. BC)

Yes. (Democritus was at least essentially wrong about the atoms and space thing upon a somewhat naive interpretation, but he was right about convention. And I say this as someone who dislikes Democritus.)