This is normal. As e-liquid ages, it can get darker. This happens due to oxidation, this oxidation has very little effect on the way the liquid tastes or the amount of vapour it produces.
If your e-liquid has gone a very dark colour and starts to taste different, it could be that you need to replace your atomiser. Over time the cotton or wick in the atomiser or coil will start to burn, once it gets to a certain point it will start to affect the e-liquid.