Full Idea
'Blue' and 'brown' are of the same kind, for the substitution of one for the other, though it may falsify the proposition, does not make nonsense of it.
Gist of Idea
Words of the same kind can be substituted in a proposition without producing nonsense
Source
Ludwig Wittgenstein (Lectures 1930-32 (student notes) [1931], A I.4)
Book Reference
Wittgenstein,Ludwig: 'Lectures in Cambridge 1930-32', ed/tr. Lee,Desmond [Blackwell 1980], p.3
A Reaction
He chooses an easy example, because they are determinates of the determinable 'coloured'. What if I say 'the sky is blue', and then substitute 'frightening' for 'blue'?