Parakeets are known for their bright and beautiful plumage; it's obvious when a parakeet's colors change. Slight shifts in shade or intensity are normal and can happen many times throughout a bird's life. But a major change to a different color, or small changes in conjunction with unusual behavior, could signal a problem.
Youngsters
Before the first molt, or change in feathers, a young parakeet will typically have a striped pattern on most of his head and dots on his wings. New feathers emerging in these areas will be mostly solid colors instead of patterned, changing the appearance of the young bird and leaving behind only a small patch of striped feathers on his head. With the exception of a few color varieties that don't exhibit these changes, it is a normal part of the growth process of a young parakeet.
Molting
Some parakeets molt more frequently than others, but it is normal for a parakeet to molt, or shed his feathers and replace them with new ones, once or twice a year. During this process, patches of new feathers emerge next to patches of old ones. The new feathers are often brighter or more vividly colored than the old feathers, which are about to die and fall out. Molting occurs in an even pattern on parakeets; for example, both sides of the head will molt at the same time and in the same pattern. If you observe a group of oddly colored feathers in only one spot, such as on one wing and not the other, the cause is probably not molting.
Ceres
At around one year of age, the cere, or pink fleshy nostril band above the beak, changes color in parakeets. This is normal and expected. In female parakeets, it changes to tan, pale blue, white or brown. In males, it changes to a bright, vivid blue, except for a few color varieties in which it changes to bright purple or remains pink. This color change helps to easily tell the difference between mature males and females. If the cere on an adult budgie changes color, it is usually a sign of disease or illness.
Feather Damage
Feathers that are damaged or dirty can appear different than surrounding plumage. A feather that is bent or broken near the shaft or has been damaged by parasites will usually look darker than the rest of the plumage. A group of damaged feathers may look like a discolored spot on the bird's body. Feather damage can occur from over-handling by humans, over-grooming by the bird and dragging or snagging on the cage bars. It can also be a result of illness or malnutrition.
Illness and Nutrition
When a parakeet is ill, other signs usually accompany feather damage or discoloration, such as a change in appetite, a puffed-up appearance or depression and lethargy. Some illnesses have few outward symptoms, however, and some birds are good at hiding signs of sickness. Liver disease, for example, can cause groups of very brightly colored feathers to appear. If your parakeet is changing color and there is no reasonable explanation for it, such as molting, or you suspect that he may be ill, contact your veterinarian. Feathers can lose their color or change color if your bird is malnourished. Discuss your parakeet's diet with your veterinarian, especially if no other cause can be found for feathers changing color.