Yes, this is completely normal. Nicotine is a reactive substance, which makes it oxidize and change the colour of the e-liquid when it is exposed to light, heat, or air, or when mixed with other ingredients in the e-liquid. Even e-liquid without nicotine and with proper storage, will slightly darken over time. We always suggest to store e-liquid in a cool, dark, and dry place away from direct sunlight to keep your e-liquid fresh.