Pure Vitamin C (L-Ascorbic Acid) is a highly effective antioxidant that is notoriously unstable. Due to its delicate makeup, L-Ascorbic Acid is vulnerable to oxidation when exposed to light, heat, or even just air, making some level of discoloration almost inevitable over time.
Fortunately, Vitamin C retains most of its potency through the first several phases of oxidation, which means it’s still both efficacious and perfectly safe to use. A champagne or straw-colored Vitamin C serum is completely normal and still provides your skin with a potent dose of antioxidants.
Here is a cheat sheet to show the level of discoloration relative to the serum’s efficacy.