Unless it have not been stored in a cool, dark place, your CBD is no less effective for its magical new hue.
If you've ever cut into an apple and left it out rather than eating it straight away, the browning of colour is due to this same enzyme. Polyphenol reacts with oxygen, and then works to oxidize polyphenols within the product.
This reaction does not alter your CBD oil or its efficacy in any way - it actually means the MCT oil used in the CBD oil is of a very high quality.
But if for any reason you're not down with this change in colour, storing your CBD oil in your fridge can help prevent it from happening.