I'd be very concerned if there's tarnish in your atty. On the other hand, the discoloration of your coil may just be the usual gunk that could build up depending on your juice. Before I re-wick, I dryburn first then take a wet toothbrush and gently brush the coil very lightly...it removes any leftover burnt gunk after dryburning, then rinse it before re-wicking it. You'll be surprised that it'll look and perform just like a fresh coil. On a different note, it does seem that your coils might not be firing evenly.