A non native speaker asked me this but I'm unsure.
Is a sentence like this correct:
"She liked the red ball; I liked the blue."
Replace blue with literally any adjective.
I'm unsure if this only works with "certain" adjectives that can be read as nouns "The Poor" "The Hungry".
"She liked fast birds; I liked slow."
There - that one there is no "the" construction even. I think in some sentences, particularly with "the" in front of the hanging adjective -- it reads as correct but maybe technically isn't?