From the W3C pages on SKOS collections I understand that concepts can be grouped in 'collections'. However, concepts can also be part of a concept scheme.
Am I right in assuming that the difference between the two is that a conceptScheme will more often be based on some theoretical underpinning or schema, while collections are more arbitrary sets of concepts?
For example, if I had a schema of a product range and customers would have a combination of those products, would I then put the whole product range as conceptScheme, while referring to the products owned by a customer as a collection?
SKOS collections are intended to represent groupings of closely-related concepts within a particular thesaurus. The SKOS Primer gives the example of the collection "milk by source animal" which contains the items "cow milk", "goat milk", etc. So it's an additional organizational feature (orthogonal to the normal concept hierarchy) for closely-related terms - the idea being that "milk by source animal" is itself not a concept - the broader concept for "cow milk", "goat milk", etc. would still be "milk".
Concept Schemes have a different intent: they are intended to capture/identify a single complete thesaurus/taxonomy, which is useful in a situation where several such thesauri/taxonomies co-exist. Items in a ConceptScheme are not necessarily a "grouping" of closely-related terms, but are all part of the same overall hierarchy of terms.
Applied to your examples: a product range schema seems to fit the 'single complete taxonomy' definition best, so that is likely best captured using a ConceptScheme.
As for products owned by each customer: I am unsure whether I would represent that using a collection at all (you could simple have an identifier for your customer and just have 'ownsProduct' relations point to each individual product). Even if you did, a SKOS collection is not the best fit: this is more situational knowledge and less to do with the thesaurus/taxonomy itself. But if you had a need for grouping certain products together in your product range schema, e.g. "Toothpastes by taste" (with subtypes 'sweet tasting', 'salty tasting', and 'minty tasting'), then that is what you would use a SKOS Collection for.