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We not only stop at red lights, we make statements like S1: "subjectively, red is closer to violet than it is to green." We have cognitive access both to "objective" phenomena like the family of wavelengths coming from the traffic light, and also to "subjective" phenomena of certain low-level sensory processing outputs. The epiphenomenalist has a theory on the latter. Your steelman is well taken, given this clarification.
By the way, the fact that there is a large equivalence class of wavelength combinations that will be perceived the same way, does not make redness inherently subjective. There is an objective difference between a beam of light containing a photon mix that belongs to that class, and one that doesn't. The "primary-secondary quality" distinction, as usually conceived, is misleading at best. See the Ugly Duckling theorem.
Back to "subjective" qualities: when I say subjective-red is more similar to violet than to green, to what does "subjective-red" refer? On the usual theories of how words in general refer -- see above on "horses" and cows -- it must refer to the things that cause people to say S2: "subjectively this looks red when I wear these glasses" and the like.
Suppose the epiphenomenalist is a physicalist. He believes that subjective-red is brain activity A. But, by definition of epiphenomenalism, it's not A that causes people to say the above sentences S1 and S2, but rather some other brain activity, call it B. But now by our theory of reference, subjective-red is B, rather than A. If the epiphenomenalist is a dualist, a similar problem applies.
I don't see how you can achieve a reductionist ontology without positing a hierarchy of qualities. In order to propose a scientific reduction, we need at least two classes, one of which is reducible to the other. Perhaps "physical" and "perceived" qualities would be more specific than "primary" and "secondary" qualities.
Regarding your question, if the "1->2 and 1->3" theory is accurate, then I suppose when we say that "red is more like violet than green", certain wavelength ranges R are caus... (read more)