Yep, tanks do that. More VG and sweeter juices do it worst but, all of them will darken slightly. Does it in even a CE4 but those are smaller, so you don't notice it nearly as much as in something like a Davide or Protank.
Need help from former MFS (MyFreedomSmokes) customers
Has any found a supplier or company that has tobacco e-juice like or very similar to MFS Turbosmog, Tall Paul, or Red Luck?
Just another voice to add to the pile of, "Yep, same here" and "it's probably the heat."
But, here's a new twist -- does anyone get a slight flavor change along with the color change? I've noticed with one of two of my juices that when the color gets darker, the flavor and throat hit also shift a just a nudge toward the harsh. Not to the point where it's something I'd complain about, just a sort of "on the edge of my mind" thing where it registers and then I can dismiss it.

Some juice darkens more than others. My juice used to darken (burn) like crazy. Was vaping around 8.5 Watts. Now I vape around 7.0 Watts, no darkening. Same tanks, same juice. As has been mentioned over and over, it's the heat, the leach back, and the type of juice. If you like it great. If you don't, reduce the heat. I found that the darker juice is harder on my wicks, and more heat with my sweet juices both caramelizes and carbonizes my wicks. Both lead to flooding and leaking eventually, more frequent cleanings, and to earlier coil death. Just my experience. Eventually, I found that the lower heat actually allowed me to experience the flavor a bit better. Of course, my taste buds have changed over six months, and I don't need as much heat, as much flavor, or as much nic. Different for everyone. My two cents.