It is obviously true that "🟩", "greenness", and "activity in the visual cortex" are not identical things. For example, "greenness" is one word containing the letter "g", but not the letter "a", whereas "activity in the visual cortex" is five words containing the letter "a", but not the letter "g", and "🟩" does not contain any letters at all.

Usually, when someone says that "🟩" is "greenness", they do not mean that the word "greenness", in a thought, spoken, or written form, is strictly identical to "🟩", or that "🟩" does not exist, and only "greenness" does. What they mean is that "greenness" is a useful label to describe "🟩". It is generally understood that labels are not the things that they are describing.

This understanding breaks somewhat when discussing illusionism - which can, at least partially, be attributed to illusionists not being very good at explaining their position.

When someone says that "🟩" is "greenness", we do not accuse that person of denying the existence of "🟩". Why then, when someone says that "🟩" is "activity in the visual cortex", do we make such an accusation?

An illusionist would not deny the existence of "🟩" (perhaps some illusionists would, Keith Frankish certainly does not). An illusionist would merely claim that so far, the most useful framework that has been discovered for verbally reasoning about "🟩" is the framework of brain configurations and activity in the visual cortex.

Some would say that the analogy presented here is unfair, because the word "greenness" generally only has one referent, whereas the phrase "activity in the visual cortex", according to illusionism as explained here, has at least two referents, namely "🟩" and micromovements in a grey, squishy, and wet object.

However, that does not make the analogy unfair. The core claim I am attempting to refute here is that illusionists somehow deny the existence of "🟩". That a term has multiple referents is irrelevant as an objection to my refutation.

Another point that could be raised is that it is not evident that the framework of activity in the visual cortex is the most useful framework in which to reason about "🟩". That point however is also irrelevant as an objection to my claim that illusionists do not deny the existence of "🟩" when they say that "🟩" is "activity in the visual cortex" - analogously to how people do not deny the existence of "🟩" when they say that "🟩" is "greenness".

It could indeed be true that there exist more useful frameworks for reasoning about "🟩". But one should understand what illusionism says before attempting to argue against it.