I am thinking along the lines of Fibergeek. I have seen it on extruded 6061 and 2011. Sometimes it shows up as dark lines straight from the anodizing tanks, and other times it doesn't show up until it is dyed, and the darker colors are worse. So, if the lines are perfectly parallel to the length of the tube, I would look at that first. Also, sometimes the effect goes deeper than the surface, so just barely removing any visible (before anodizing) lines may not be enough.
If there are streaks that only closely follow length of the tube, it may be something else.
Do you know the alloy, and what kind of tube it is?
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