A vitamin C serum that has become noticeably darker deserves a closer look, but its colour cannot tell you exactly how much vitamin C remains. Compare the change with that product's original appearance and storage instructions. A pale-yellow serum on its first day and a bottle that has gradually turned brown are different situations.
Why a bottle can darken before its expiry date
Korean skincare brand COSRX explains that pure vitamin C can oxidise as it encounters air, light and changing storage conditions. Its description links progressing oxidation with a darker yellow colour that can eventually become brown. That gives a reason to pay attention to a colour change; it does not provide a chart for calculating the strength left in a bottle.
An expiry date and the condition of an opened product answer different questions. A date printed on the box cannot tell you where the bottle has been sitting since you opened it. For the same reason, remembering only that you bought it recently is less useful than knowing when you first opened it and how it has been stored. Our guide to EXP, MFG and the open-jar symbol explains which dates and opening periods to keep track of.
I would take a quick photograph when opening a serum whose manufacturer warns about discolouration. Use the same lighting when comparing it later. Differences in lighting can make the same serum look darker or warmer.
COSRX Vitamin C 23 has specific storage instructions
For The Vitamin C 23, COSRX specifies upright storage in a refrigerator at 3–10°C after opening. It recommends using the product within two months of opening and keeping it away from heat and direct sunlight.
Those instructions make the practical commitment clearer. Buying several bottles at once is less attractive if you are still working through the first. Writing the opening date on the box, closing the dropper promptly and putting the bottle back in its designated place are more useful habits than trying to rescue an uncertain bottle afterwards.
This is a rule for that product. A different vitamin C serum may have different instructions. Do not transfer the two-month period to every product with vitamin C in its name, or assume that every serum belongs in a fridge.
What I would do with a bottle that has changed
Start with its own instructions and a reference for the fresh product. If it has substantially darkened, pause use while asking the manufacturer whether that change is within the expected range. Include the product name, batch information, opening date, storage conditions and a clear photograph. A picture of an unidentified orange drop gives them much less to work with.
I would not add water or another serum to make the colour look lighter. That changes what is in the bottle without answering whether the original formula is still suitable to use. Nor would I treat a familiar smell as proof that everything else is unchanged.
If the manufacturer advises replacing it, or the labelled use period has passed, finishing the bottle for the sake of avoiding waste is a poor bargain. Next time, choose a bottle you are likely to finish within its stated opening period.
A vitamin C serum should fit the way you actually use skincare. If refrigeration and a short opening period repeatedly leave you with unfinished bottles, that storage burden belongs in your next purchase decision alongside the concentration on the front.