Yes, it's normal. E-liquid just left sitting in a bottle will change color as it ages. It will usually get darker and often develop a different flavor over time. Some call this steeping, some say aging.
Either way, it's a good thing to remember for when you get a juice you don't like when you first try it. Just put it away for a week or more, and try it again.
Also, in your tank, the juice will get darker still due to the burnt juice coming off the coils and washing back into the tank. It's really nothing to worry about, but it is a good reminder that our tanks, clearos and other toppers need a wash out and a clean every few fills to keep things working right.