It is a problem because it shows poor feather growth. Often the purple shows up in "bands," and those bands are unwanted. I don't know what causes it, I just know it's not a good thing, health-wise, for the feathers.
Now if a bird has a green sheen that may change a slight blueish purple from time to time, that's fine, but the bird needs to be green most of the time. Check the feathers.