In the proposition “purple is [not] bitter” it seems clear to me that “bitter” is being used adjectivally. Imagine someone with a variety of synaesthesia that makes them perceive bitterness whenever faced with something purple; then I would say that for them purple is bitter. (In much the same sense as we might say that quinine is bitter.) For most people, colour perception and taste perception are not linked in any such way and therefore purple is not bitter.
This seems reasonable to me. In any case the argument wasn’t really about whether purple is bitter, but whether the sentence “purple is bitter” has any meaning at all. In fact is obviously has at least one meaning (which you mention here) and potentially several.
In the proposition “purple is [not] bitter” it seems clear to me that “bitter” is being used adjectivally. Imagine someone with a variety of synaesthesia that makes them perceive bitterness whenever faced with something purple; then I would say that for them purple is bitter. (In much the same sense as we might say that quinine is bitter.) For most people, colour perception and taste perception are not linked in any such way and therefore purple is not bitter.
This seems reasonable to me. In any case the argument wasn’t really about whether purple is bitter, but whether the sentence “purple is bitter” has any meaning at all. In fact is obviously has at least one meaning (which you mention here) and potentially several.