There are two types-of adjectives that can function as nouns. The first type includes adjectives that refer to groups of people by specifying some quality, including nationality, that they have in common, e.g.:
The rich. get richer and the poor get poorer.
This wing of the hospital is reserved for the critically ill.
The British and the French do not always understand each other.
These adjectives functioning as nouns always have generic reference, do not inflect for number or the -s genitive, and require a singular verb.
There are other cases where adjectives may be said, function as nouns at a superficial level, but it would be more accurate to say that they represent or substitute nouns (i.e. as pronouns), e.g.:
Do you prefer Irish coffee or Turkish (coffee)?
We ordered mashed potatoes, but he served us boiled (potatoes).
The red dress is prettier, but the green (dress) suits her better.
The noun head in such sentences is elliptic, i.e. retrievable from context (most often in the preceding clause); it can have specific or generic reference and be singular or plural. These cases of ellipsis can thus be distinguished from the adjective-nouns described above. With the first type, referring to groups of people, only a noun such as people or human beings could be inserted:
Rich people get richer and poor people get poorer.
This wing of the hospital is reserved for people who are critically ill.
British people and French people do not always understand each other.
With the second type, although it is sometimes possible to insert a vague word like thing, there is often no corresponding noun:
The best (thing) is_yet to come.
In science one sometimes has to distinguish the unknown (facts, things) from the unknowable (facts, things).
but
It's not always good to venture into the unknown.
He went from the sublime to the ridiculous.