for why the wavelength of red feels from the inside like a built-in quality of the world rather than a perception generated by a mind in response to a stimulus.

For any agent, self-reflection has to bottom out somewhere, since working memory and cognitive capacity are finite.

That said, some meditation practitioners report being able to notice "the arising and passing away" of individual sensations, so it may be that this is just a matter of training rather than an essential feature of consciousness.