I’m with @ringling on this one. There’s really no worry. Some flavorings just turn more brown than others when heated by the coil(s). Stay at a reasonable ohm on the coils and vape on!
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Would you please elaborate on the sweeteners/EM/Brown Sugar etc.?
I’m having this issue currently with my mixes and I suspect it to be the sweetener I’m using. (Sucralose)
Sweeteners tends to burn on coils when they are heated.
A lot of USA style juices that are super sweet tend to do this.
Any flavourings that you find that turn dark over time will also gunk up coils.
to add to these great insights, sweetener is the worst, and many premium juices that use it - also some flavors… example, i have 3 different Caramel flavors from different vendors that i use in my mixes now, with only 1 of them i will very quickly get very brown wicks… the others don’t behave the same… the juice inside the tank (RTA) will also turn slightly brownish … no worries
well… this is normal… nothing to worry about…
the brownish color you see is mostly by any kind of sweeteners you juice has inside…
but "HEY M8!!! i didn’t add ANY sweeteners inside…! " keep in mind that flavors have some sweeteners inside…
if you’re making a clone of your favorite bought juice, keep in mind that it’s always a clone…
in the original recipe let’s say … the company used this and that and something else… while in a clone “this” might be something else in a different % or smthing and contains some sweeteners or whatever… while always remains a clone… “clone tastes really closely to an original recipe but it’s NEVER the original” in case you want to relate them… the author of a clone is trying to copy the TASTE first and secondly the original ingredients… for example you could use TPA strawberry in X percentage… while the original used the FA one at Y percentage…taste goes the same? yes we keep the TPA strawberry (just a completely random example)
another reason that you can see brown color could be burnt wick… when wick gets burnt starts getting brown first, and then goes black… oh well… after this it gets fire 
other reasons could be coloring inside flavors (for example i remember a company for some flavors was using coloring inside… some of them were red, another one was blue, another one was green etc)
just get the wick out… dry burn your coils… clean them, see the wire conditions and change cotton… if the wire seems being in bad condition… change that too 