Cockatoo with yellow/brown feathers by: The Avian Vet
Your bird is probably dirty. Nicotine stains will cause this if you smoke. Nutrition can also contribute. Make sure your bird is on a pellet diet. Harrison's or Roudybush pellets are best.
You can also bathe your bird. There are some bird shampoos that will clean your bird safely. Finally, I recommend that you have your bird seen by an avian veterinarian. You should do this annually for well bird checks and vaccines.