Are you a repackager
On this page11
Start from the definition
Marketed or represented by the producer or repackager as suitable for use in making art. If you are marketing it and you are not the producer, the remaining word is the one that applies to you.
Decanting is the clearest case
Buying a litre of paint or primer and selling it in 15 or 30 mL bottles under your own label is repackaging in the ordinary sense of the word and in the statutory one.
Mixing is a harder case and a bigger one
A wash you blend from a paint, a medium and water is arguably a new formulation rather than a repackaging, which does not get you out of the definition. It puts you closer to producer than to repackager.
Neither is a way out
Both words are in the definition and both carry the same label contents. The distinction matters for whose toxicological review you can rely on, not for whether a review is needed.
Relying on the original review is the real question
A producer's determination is about their formulation in their container. How far it carries to your decant of it is a question for the toxicologist who did it, and it is worth asking in writing.
The label names you, not them
Subsection (b)(5) requires "the name and address of the producer or repackager of the art materials and an appropriate telephone number". That is your name and your telephone number on the bottle you sold.
And the duty to update follows you
(b)(6) requires new hazard information to be "incorporated into the labels of such art materials that are manufactured after 12 months from the date of discovery". A repackager who stopped paying attention after the first print run has a live obligation.
Selling at a convention is still selling
The statute turns on marketing a substance, not on the size of the operation or the channel. A table at a games convention and a web shop are the same act.
Giving it away is a different question
Marketed or represented is the hinge, and a free sample handed out to promote a product is closer to marketing than most people assume. Worth thinking about before a convention goody bag.
Which is why this page exists at all
Nobody in this hobby thinks of themselves as an art materials manufacturer. The statute does not ask what you think of yourself, it asks what you market and under whose name.