Vitamin C Serum Color Change

Regarding your question about the Stabilized Vitamin C (Natural-C Glycol™) Serum changing to a dark yellow color:

While using the product after it has changed color is generally not harmful, a significant change to a dark yellow or brownish color indicates that the Vitamin C (L-Ascorbic Acid) in the serum has oxidized and degraded.

When Vitamin C oxidizes, it loses its beneficial properties, such as its ability to brighten skin, act as an antioxidant, and stimulate collagen production.

As the previous reply mentioned, if the color is very dark, similar to Betadine, the product has likely lost its effectiveness. In this case, it is best to discard the product, as applying it will not provide the desired benefits.