Black streaks are mold/algae.
Brown? Question, is your gutter clean? Been parked under an oak or walnut tree? I get a muddy brown streaks from my black walnut tree if the gutters extensions are plugged up.
Brown, makes me think of rust, but as you say, there are aluminum screws, so not rusty screws. I would remove a screw or two, and check the color of the threads. If they are rusty looking, you have a water leak, at the screw. Remove each screw apply a tiny amount of sealant and reinstall.
Can you post a close up picture at the source?
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