The change in color is normal as the e liquid leaves resins on the coil as it heats up. When you refill, the resins from the prior liquid that is stuck on the wick, merge with the liquid. You can avoid that by changing wick and dry burning.
Some liquids are famous for gunking up the coils more than others.
That's what I've been doing, changing wicks everyday or every other day. It's a PITA. I have to change coils around every 3 days due to the sweetener in my juice and that's a bigger PITA because half the time I break the ceramic cup. I need a stock pile of those. Don't get me wrong. I love my Sophia's but they are requiring a lot of rebuilding time. It's probably my DIY juice but it's my all day juice and don't like anything else. There's nothing better than a Sophia with a fresh setup and full tank of fresh juice. That's why I put up with the back wash.
