dont be conserned this is normal, after however many times you heat up and cool down the coil on the ce2 the juices seem to start to darken towards the bottom or middle of your fill I havent noticed any change or major change in the fluid, I vape mine down to the very last drop and by that time they sometimes can look absolutly really dark by that time, it is only a reaction to the heating and cooling process though I believe. Good Luck though