Some things I would ask
Are you dripping enough juice to keep the wick saturated?
Have you used any form of sweetener/EM/Brown sugar/molasses/cotton candy etc in your mix?
But over all there is no need to worry.
I find custards tend to colour my wicks faster than any of my other recipes.
J
It depends on what flavors you use - dark “brown sugar”, cocoa, chocolate etc will gunk up your and color the wick fast! 
Another question, when you re wick, do you do a “cleaning burn” and do you rinse the addy? I ask because my son never did and I was surprised to find a really dark brown around his terminal screws,so bad that I did take it apart and give it a good bath.
It depends entirely for me how long I’ve been using the coil.
If I’ve been hammering my Vape for a few days and then want to change flavour I will take my wick out and then fire until glowing for about 5-10seconds and then rinse under a faucet to get any gunk off.
Then quick dry burn to dry off and rewick.
If it’s not been used very much then I’ll just dry burn and rewick for a new flavour.
I’m definately by no means a pro though.
J
That’s the same here on a normal change, every once in a while though I break it down, taking the screws out and all, and give them a good cleaning. Sometimes they are really bad.
If I’m doing a full coil change then I’ll generally rinse under the faucet with warm water to get any residual gunk off.
J
Yup… normal for us custard, vanilla, coffee, chocolate, coffee, and sweet juice junkies. 
Holy s**t lol.
That’s looks seriously dark on that pic. Lol.
J
that’s the thing though, i’m not using custards, tobaccos, or anything dark… all of my juices that i am currently using are 99% translucent
If you use citric acid, sweetener or something like it, it will do the same…
Ok… just to make sure you understand… the juice I’m using right now is not dark. But, when the coil
heats it, the cotton turns dark because there is vanilla, custard, and sweetener in the juice.
Can you share your mix recipe ingredients, so we can see what you’re vaping?
Looks like a HexOhm?
i didn’t even think about it but my one juice orange cream has cream flavor in it obviously, so i guess that’s where it’s coming from…
but as of right now what i mixed up are as follows, all being 80/20 VG/PG:
Strawberry Fruit Circles- fruit circles flavor from flavor west with a touch of strawberry natural from TFA, probably at a 2/3 fruit circles 1/3 straw
Orange Cream- straight orange cream flavor from TFA i believe
Yumberry- straight yumberry flavor from TFA i believe
My guess is: It’s probably Fruit Circles from FW that does it
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!
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 