Full Idea
If grammar says that you cannot say that a sound is red, it means not that it is false to say so but that it is nonsense - i.e. not a language at all.
Gist of Idea
Grammar says that saying 'sound is red' is not false, but nonsense
Source
Ludwig Wittgenstein (Lectures 1930-32 (student notes) [1931], B IX.6)
Book Reference
Wittgenstein,Ludwig: 'Lectures in Cambridge 1930-32', ed/tr. Lee,Desmond [Blackwell 1980], p.47
A Reaction
I am baffled as to why he thinks 'grammar' is what prohibits such a statement. Surely the world, the nature of sound and colour, is what makes the application of the predicate wrong. Sounds aren't coloured, so they can't be red. False, not nonsense.