from my personal experience,
most e-liquid changes into a darker color and become less viscous, as the tank gets lesser, so is the flavour of the of the liquid. It doesn't taste as good as when I just filled the tank full.
for a darker color, 2 explanations could possibly be the answer:
1. Liquid has steeped due to the repeated heating and cooling of the liquid contained in the atomiser. Especially RBAs.
2. Caramelized "residue" backflow into the tank, I get this alot with the taifun GT and fogger due to it's design, the location where the wick/cotton is being placed horizontally therefore promotes backflow. On the other hand, kayfun actually controls the backflow very well as it is designed in such a way whereby liquid only flows in and hardly flows out, there is little or no "exchange" involved.
There may be more explanations for this but this is what i've personally experienced.
Tank low = flavor lesser, liquid darker
Tank low = Sometimes liquid tastes better, sometimes worse.
I guess it all depends on the characteristics of the ingredients used in the mixing of the e-liquid
care to share what liquid did you use? I'm curious and would like to try a liquid that actually turns lighter as it gets "steeped", I haven't get that before.