GHK-Cu is blue. Properly, vividly blue, and that is not a contamination problem, it is copper.
The peptide is three amino acids wrapped around a copper ion, and it was found in human plasma, where the amount present falls markedly with age. That observation launched decades of research into what the complex is doing there.
Structure and the copper complex
Glycyl-histidyl-lysine, three residues, bound to a copper ion. The histidine does most of the coordinating.
The copper is not an impurity or an additive. It is part of the molecule, and the complex behaves differently from the bare tripeptide, which is why the certificate describes the complex rather than the peptide alone.
Why it is blue
Copper complexes absorb light in the red part of the spectrum, so what reaches your eye is blue. The same chemistry gives copper sulphate its colour.
This is the one compound in the catalogue where a coloured powder is correct. If a certificate for GHK-Cu describes the appearance as white, that is worth a question, because a properly formed copper complex is not white.
The question researchers are asking
Plasma levels fall considerably between young adulthood and later life. That correlation is what drew attention, and the published work since has looked at wound models, extracellular matrix remodelling and the regulation of genes involved in tissue repair.
Copper is essential to a number of enzymes and toxic when handled badly, so a molecule that carries it in a controlled complex is interesting in its own right, whatever else it turns out to do.
Handling and light sensitivity
Soluble in water. Store the sealed vial at minus twenty and keep it out of the light, which matters more here than for the colourless peptides, since a compound that absorbs visible light has more opportunity to be affected by it.
Solutions retain the blue colour. Fading is worth noticing.
What to check on your certificate
Appearance should describe a blue powder. Copper content should be stated, since the complex is only right if the stoichiometry is right. Then the batch reference against the vial.
Regulatory position in the UK
Not a licensed medicine, supplied as a laboratory reagent for in vitro research.