In "Collected Philosophical Papers Volume 2: Metaphysics and the Philosophy of Mind", Elizabeth Anscombe wrote the following:
I always hated phenomenalism and felt trapped by it. I couldn’t see my way out of it but I didn’t believe it. It was no good pointing to difficulties about it, things which Russell found wrong with it, for example. The strength, the central nerve of it remained alive and raged achingly. It was only in Wittgenstein’s classes in 1944 that I saw the nerve being extracted, the central thought “I have got this, and I define ‘yellow’ (say) as this” being effectively attacked. –At one point in these classes Wittgenstein was discussing the interpretation of the sign-post, and it burst upon me that the way you go by it is the final interpretation. At another I came out with “But I still want to say: Blue is there.” Older hands smiled or laughed but Wittgenstein checked them by taking it seriously, saying “Let me think what medicine you need… . Suppose that we had the word 'painy’ as a word for the property of some surfaces.” The 'medicine’ was effective, and the story illustrates Wittgenstein’s ability to understand the thought that was offered to him in objection. One might protest, indeed, that there is this wrong with Locke’s assimilation of secondary qualities to pain: you can sketch the functioning of “pain” as a word for a secondary quality, but you can’t do the reverse operation. But the 'medicine’ did not imply that you could. If “painy” were a possible secondary quality word, then wouldn’t just the same motive drive me to say: “Painy is there” as drove me to say “Blue is there”? I did not mean “'Blue’ is the name of this sensation which I am having,” nor did I switch to that thought.
I find this passage a bit obscure, and I was hoping someone could flesh out a bit more exactly what argument Wittgenstein was making here.
EDIT: I should clarify what I'm looking for. I understand what she means by "the way you go by it is the final interpretation." What I don't understand is the argument being made in "Suppose that we had the word 'painy’ as a word for the property of some surfaces."