The reason why your cartridge looks darker is that the bio-compounds in the oil have started to oxidize. Oils in these cartridges when they are exposed to light will start the oxidization process in the cannabis oils, this process will cause them to darken and degrade. Don't worry, the darker oil is still safe to consume, but it will be less potent due to the oxidation. 

To reduce the oxidation process of cartridges, you should store your cartridges in a cool and dark location like a drawer or cabinet.