At another site some guy said it was cuz of the juice sweetner or caramel. Etc.
If it was from some sugars caramelizing, it would happen gradually with use, and would be more even across the wick. It would also likely be darker and less red.
You'll find plenty of folks that will say it is just caramelizing sugars. If you want to know for sure, set it aside for a couple days without heating the coil. It will not caramelize without heating, so if the color gets much worse with a couple days sitting, it is rust.
My bet is rust. I've seen that pattern and color of wick on more than one atomizer. It has always been clones. It has always come from either the negative screw holes, or the deck right by the posts. When it was from the screw holes, I scrubbed out the holes, cleaned and dried the atomizer, replaced the screws with good SS screws, and the problem went away.