How old is your bird and what kind of diet are they on?
There are a few possibilities.
When a tail feather experiences trauma during a vital growth stage, such as falling during a flight attempt, it can cause disruption in the growth that can cause physical anomalies or coloration changes. The next time the bird molts, this will be absent from those feathers.
As
@Shezbug mentioned, though, it can also be other stresses during the growth of the feather. They usually manifest as "stress bars" which is a weaker part of the feather. This can be from emotional or physical trauma (not direct trauma like I stated above) or more likely, from lack of nutrients during that stage of the feather growth.