If you have ever bought a vitamin C serum and watched it slowly turn from clear to yellow to orange, you probably told yourself one of two things.
You told yourself it was fine. Normal oxidation. Nothing to worry about.
Or you told yourself you had not been using it fast enough. That you should store it better. Refrigerate it. Use it before it had a chance to turn.
What nobody told you is that the color change is not a storage problem. It is not a usage problem. It is a chemistry problem. And it means the ingredient you were paying for was already gone.