The darkening of e-liquid containing nicotine is a common occurrence and does not necessarily indicate that your e-liquid has gone bad or expired.
When exposed to air, nicotine oxidizes, creating a darker yellow or orange hue. If an e-liquid is opened and exposed to oxygen, the oxidization process can happen fairly quickly. Once you’ve opened your e-liquid, try to use it promptly, rather than setting it aside for long periods of time. Oxidization is also accelerated by direct heat or sunlight, so it’s best to keep your e-liquid in a dark, cool environment.