A formulator asked us for "the stronger anti-ageing peptide" for a clear 30 mL serum. Our first question was not about efficacy data - it was whether the finished product had to stay colourless through a 12-month shelf life.
The question behind the question
GHK-Cu is a copper-binding tripeptide. The complex has a visible blue colour, and copper ions interact with chelators, some preservative systems and higher pH. Acetyl Hexapeptide-8 is a water-soluble peptide used in leave-on expression-line products and is easier to place in a transparent aqueous base. Neither is simply "stronger"; they fail in different places.
A worked example you can check
Assume a brief fixes the active at 0.05% in a 100 kg batch. That is 50 g of active for the whole batch. Filled at roughly 30 g per 30 mL unit, 100 kg gives about 3,333 units, so each unit carries about 0.015 g (15 mg) of active. These numbers follow only from the stated assumptions - they are not a recommended inclusion rate - but they show what a supplier needs to know: the total charge, the unit count and the fill weight.
Now change one assumption: if the inclusion level is 0.1% instead, the charge doubles to 100 g and the copper colour contribution roughly doubles while the rest of the formula stays the same. That is usually the moment a transparent serum brief turns into a tinted one.
Where each one tends to cause trouble
With GHK-Cu we ask about chelators such as EDTA, the preservative, the pH window and the packaging. A blue cast that is acceptable in an opaque airless bottle can be a rejection reason in a clear glass dropper.
With Acetyl Hexapeptide-8 the questions are about solubility in the finished matrix, the preservative system and whether a leave-on product gives the formula enough contact time. A rinse-off format is an entirely different brief.
How we handle the sample stage
We would rather send two small samples than one confident recommendation. Put both into the real base with the full preservative system, then check colour, clarity and pH after freeze-thaw cycles and accelerated storage. A measurement in pure water on day one answers none of those questions.
If colour stability rules out the copper complex, we say so at the sample stage. If the copper complex is part of the concept, we work backwards from the packaging and the preservative system instead of trying to hide the colour.
What to send for a comparable quotation
Dosage form, pH window, preservative system, chelators present, target fill size and batch size, plus the documents your QA team needs. With those, a quote can name the grade and the pack format instead of a generic "cosmetic peptide".
The short answer
Choose by the failure mode you cannot accept. If the product must stay colourless and clear, start with Acetyl Hexapeptide-8. If the copper complex is part of the concept and the packaging can carry its colour, start there and design the preservative system around it. Either way, ask for a sample COA and a chromatogram before the specification is locked.