First of all, it's not oxidization. It's not due to a chemical reaction with oxygen in the air that causes the carbon fiber color to change. It's a reaction to the UV radiation from the sun that causes the shift in color. To be more specific, it's not the carbon fiber themselves that you're seeing change color but the resin epoxy used to keep it all bonded together that's sensitive to UV radiation and also moisture. Although, moisture usually isn't an issue since most carbon fiber farkels you'd get on a bike are sealed and weather proofed against moisture. The hard part with UV proofing the carbon fiber is that the cures for this tend to involve using a UV screening paint. Which then means you're not going to see the carbon fiber weave anymore which is the whole purpose why people put it on their bikes in the first place. Most professional usages of carbon fibers tend to use full on sealant and paints to protect the epoxy from UV radiation and seal it from water. Once the color starts turning, there isn't anything you can do to reverse or prevent it. Prevention itself would just be painting over it which would defeat the purpose. Also, the carbon fiber on your exhaust isn't structurally holding anything so there aren't any functional issues of having it face and change from UV radiation. Your exhaust wont fail and it probably wont even really degrade enough to fall apart as that would take decades without much force on it. It just may not look as nice as it could but should still be fine.