In act 1 scene 5 feste says to Olivia, "I wear not motley in my brain." what does he mean?

When Feste says "I wear not motley in my brain" in Twelfth Knight, he is saying that though he may look like a fool (as fools wear "motley"), his brain is not impaired and he is a keen observer.

Feste the fool  isimplying to his mistress olivia the fact that what you percieve on the outside is not always what is inside following the known term 'never judge a book by its cover'. The fool is therfore demonstrating the lack of foolishness within himself and he is the most logical chaacter