Is "something red" an example of ellipsis or adjectival positioning

“Something” is a compound determinative. There is a constraint on the position of certain modifiers called 'restrictors' that can be used to modify such compounds. The restrictors (adjectives or certain nominals) cannot occur in pre-head position because of the fused nature of the construction and are forced into post-head position, with only "else" allowed between them and the head:

“Nothing (else) significant”; "everything gold", "somewhere beautiful”, "nowhere special", "somebody rich".

It's both ellipsis (via Whiz-Deletion), and adjective positioning.

Something is a compound of the indefinite quantifier some and the generic neuter noun thing.

Quantifiers are determiners, and determiners precede adjectives, and adjectives precede nouns.
So any adjective like red would have to appear between some and thing; i.e, some red thing.

But something is glued together, so red has to go after it; i.e, something red.

This is exactly right for a reduced relative clause construction, i.e, something (that is) red.

What all this means is that using an adjective after a noun is not just a matter of adjectives

-- except a few, like galore, which must follow any noun and can't be a predicate adjective --

• There were costumes galore at the ball.
• *There were galore costumes at the ball.
• *The (number of) costumes at the ball was/were galore.

but rather a matter of the constructions involved.

If an adjective can be interpreted as a predicate in a reduced relative clause, it can go after a noun.
Contrariwise, using an adjective after a noun marks it as a predicate in a reduced relative clause.