Kastrup: Idea of the World

How does it suggest a truly external, independent reality, exactly? How exactly is it more consistent, supposedly, with a truly external reality, as opposed to an entirely unknown objective reality filtered unconsciously through a subjective lens?

Of that, I don't think you have any answer or evidence.

And no, "different wavelengths of light interact with retinal cone cells" isn't satisfactory evidence of anything.

You have to explain how retinal cone cells fundamentally work to translate raw light into subjectively perceived shades of colour.

Retinal cone cells send an electro-chemical signal.  A signal which contains limited information.  As a physical pattern channeled by the optic nerve to the brain, these signals are called an afferent stimulus.   The meaning of the inbound vision is not translated from the signal.  The meaning is detected and mirrored by the mind, as a result of the signal and the context of the agent embedded its environment.

 Afferent neurons are sensory neurons that carry nerve impulses from sensory stimuli towards the central nervous system and brain, while efferent neurons are motor neurons that carry neural impulses away from the central nervous system and towards muscles to cause movement.

 

Current research is aimed in one direction and not adapted to reciprocal activity.  While the search for the neural correlates of conscious is productive, the real discoveries will be when focused on the mental processing of information.  Instead of NCC physical functionality, science can observe how minds can relate current stimuli (including stimuli about perceived future events) to a database of personal experiences.  And from the data interchange build information objects that can change real-world probabilities.