Serum Color Change: From Greenish to Brownish

The color change you are observing in your serum—shifting from a greenish tint to a brownish one—is a common occurrence when working with complex botanical extracts and active ingredients. Based on your formula, here is an analysis of the likely causes:

1. Oxidation of Polyphenols (The Primary Cause)

The most likely culprits for the browning are Micromul™ EGCG and Calendula Extract (Water Soluble).

• EGCG (Epigallocatechin Gallate): This is a highly potent antioxidant but is extremely sensitive to oxidation. When it oxidizes, it turns from clear/light to orange or dark brown.
• Calendula Extract: As you noted, this extract contains lutein and other phytochemicals that are sensitive to temperature and light. The raw material itself is a dark brown-red liquid. When diluted in a serum and exposed to air/room temperature, it can contribute to a color shift as it stabilizes or degrades.

2. Interaction with Copper Peptides

Your serum contains Pep®-GHK2CU1-10000, which provides the initial "greenish" or blue-green color.

• Catalytic Effect: Copper ions can act as catalysts for the oxidation of phenolic compounds (like those in EGCG and Calendula). This means the presence of the copper peptide might actually be accelerating the browning of the other antioxidants in your formula.
• Compatibility: It is generally recommended to avoid mixing high concentrations of Copper Peptides with strong antioxidants like EGCG in the same formula, as they can sometimes neutralize each other or cause stability issues.

3. pH and Stability

• Niacinamide (Safe-B3™): This ingredient is most stable at a pH of around 6.0.
• EGCG Stability: EGCG is much more stable at a slightly acidic pH (around 4.0–5.0). If your serum's pH is closer to 6.0 (to accommodate Niacinamide), the EGCG will oxidize much faster.
• Liquid Azelaic™ (Azeloglycina): This also performs best in a specific pH range (typically 5.0–7.0).

4. Storage and Packaging

• Dropper Bottles: Even in a dark bottle, every time you open a dropper, you introduce fresh oxygen into the bottle. This "headspace" oxygen is enough to trigger the oxidation of sensitive ingredients like EGCG and Calendula.
• Temperature: Since the Calendula Extract and EGCG are heat-sensitive, leaving the bottle at room temperature significantly speeds up the chemical reactions causing the color change.

Recommendations:

1. Refrigeration: Continue to store the finished serum in the refrigerator. This is the most effective way to slow down the oxidation process.
2. pH Check: Measure the pH of your serum. Aiming for a pH of around 5.0–5.5 may provide a better balance for stability, though EGCG may still brown over time.
3. Packaging: If possible, switch to an airless pump bottle. This prevents air from entering the container during use, which is critical for preserving EGCG and botanical extracts.
4. Formula Separation: You might consider making two separate serums: one for the Copper Peptides and another for the botanical antioxidants (EGCG/Calendula), to prevent the copper from catalyzing the oxidation.
5. Antioxidant Protection: Adding a water-soluble antioxidant stabilizer (like Sodium Metabisulfite, though not in your current list) can help, but refrigeration and airless packaging are the best "clean" solutions.

The browning doesn't necessarily mean the serum is "spoiled" immediately, but it does indicate that the antioxidant potency of the EGCG and Calendula is decreasing.