Dark feather discoloration could indicate a Vitamin A deficiency. If you bird eats mostly seeds, I recommend gradually weaning him over to a pelleted diet, such as Harrison's bird diet.
Some foods that are high in vitamin A include carrots, sweet potato, kale, spinach, red pepper, mangoes, and apricots. Try sprinkling a little chili powder or paprika, both containing vitamin A, over vegetables for a little added boost and change of pace.