SUBSTANTIVIZED ADJECTIVES

Adjectives can often become substantivized (become nouns). In such cases they are either wholly (fully), or partially substantivized.
I. Wholly substantivized adjectives(converted into nouns) acquire allthe features of nouns: number, case, articles. They denote:
1) Class nouns:a relative, a native, a savage, a criminal, a white, а black,
a romantic, a cynic, a hopeful, a liberal, a conservative, a radical, a weekly,
a daily, a monthly, a classic, a comic, etc.
anative — natives — thenatives’ huts
a relative — relatives — the relative’s visit.
Almost a hundred hopefulsstood in a queue outside the theatre.
You’re an incurable romantic,that’s what you are.

2) Names of nationalitiesnot ending in– ch, – sh:
a Russian, an American, a German, an Italian, a Greek.
a Greek — Greeks — the Greek’s cooking
a Swiss – two Swiss
Here also belong names of languages:English, Russian, German, etc.
Russian is a much more difficult language than English.
There are so many Englishes in the world.
3) Names of colours

Black suits you best.
I like bottle green.
When used in a general sense they are treated as uncountables (no plural, no “a”), when denoting shades of colours they are treated as countables and may be used with the definite article and in the plural.

The picture was in greensand bluesand the most glorious of yellows.