The lot changed and nobody said
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What happens
An upstream lot changes, or a second source is used, and the colour or the scent shifts. The customer discovers it in a finished batch of product they cannot sell.
Why it is worse here than elsewhere
Your customer has built their own product around yours. A shift in your material is a shift in theirs, and they absorb the cost of the batch it ruined.
Lot marking is the whole mitigation
A lot number on every jar and a record of which upstream lot it came from. It does not prevent the shift; it lets both of you see it coming and talk about it.
Say it before they find it
A note that this lot differs, with a sample offered, converts a relationship-ending surprise into a reason to trust the supplier. It costs an email.
Keep retained samples across lots
Not just the current one. Comparing lot to lot is only possible if the earlier ones still exist, and the question always arrives after they would otherwise have been thrown away.
The identity question hides here too
A second source with a different composition is a different ingredient, even under the same colour name. If the colorants differ, the approvals may differ, and the customer's label may now be wrong.
Which makes composition records part of the product
What was in each lot, from which supplier, with what documentation. For a business selling ingredients into other people's cosmetics, that file is the business.
A shade name is not a specification
Two suppliers selling "Antique Gold" are not selling the same thing, and neither are two of your own lots if the source changed. The composition is the specification and the name is a label on it.
Offer a reference set
A swatch card or a small sample of the current lot, kept by the customer, turns a disagreement into a comparison. It also tells you who your serious customers are, because they are the ones who ask for it.