The discoloration of the tank juice you are seeing comes from having a crud build-up on the coil wire and wick. It is not dangerous or harmful. What is actually happening is the sweetener in the juice is being caramelized at the coil from the heat. If it annoys you, change the coil and it will go away... until the next time. If the flavor and vapor production are still good on that coil, no harm is done in ignoring it. Personally, I view that as time to change wicks and burn my coils clean. I, however, build my coils, and I can reuse the coil wire and replace a wick for a penny or two. Buying prefab coils is more expensive. For me, I just vape the tank empty, disassemble it, pull out the old wicks and discard, then put the tank base back on the mod and fire the coils until the wires glow cherry red again and all the gunk burns off. Then I scrape the coil clean with my cotton hook to get the brunt carp (that's a fish) off; take the whole tank in pieces and wash it in hot, soapy water; let it dry; re-wick with a new piece of rayon; reassemble, fill and vape.
I am not certain-- I don't use prefab coils-- but it is probably possible for you to do something similar with your coils; pull the wick, burn it clean, wash it, re-wick and reuse. Many of the prefabs can be done like that and somebody built it from scratch somehow. I'm sure somebody will be along sooner or later who's tried it with Triton coils who can tell you how, or of it's worth the trouble.