What is the significance of the yellow note in Chestnut Tree Cafe?"There came into it—but it was something hard to describe.It was a peculiar,...

My interpretation of the yellow note is that it has come
to signify the cowardice and betrayal that have now combined to destroy Winston.  He and
Julia both discuss the fact that they betrayed each other, that under the incredible
torture they both gave up the other, that in the end "All you care about is yourself." 
Because of this, the affection he felt for her, the only really true thing in his life,
has now become as cracked and yellow and empty as everything else around
him.

The "jeering" note serves to remind him of this, to
remind him of his deepest inner cowardice, that which betrayed all the hatred he felt
for the party, all the bravery he thought he possessed that allowed him to consider such
things as helping the proles to revolt, etc.  The music is not just despicable or empty
and tinny now, it is yellow and it jeers at him, mocking him as a human
being.