I am playing La Fille aux Cheveux de lin by Debussy and I am trying to work out how to play in a really melancholy tone to express the piece full but it never turns out the way I want. Is there any trick to bending tone to fit certain emotions or anything that could help?

Maybe changing the vowels you "pronounce" when playing, just like the opera singers do.

If you can change the tone of your voice, you certainly can do it to your tone, just that you won't use the vocal cords.

again, I agree with Zevang, listen to a singer do it...don't ever play anything like a flute player, always think of each note as a word, vowel, syllable, etc...

shape your tone..it will work , you will see..