red vs scarlet

red and scarlet both are nouns.

Word	Noun	Adjective	Verb	Adverb
red	Yes	Yes	No	No
scarlet	Yes	Yes	No	No

As adjectives, red and scarlet are synonyms defined as:

red and scarlet: of a color at the end of the color spectrum (next to orange); resembling the color of blood or cherries or tomatoes or rubies

Other synonyms of red include blood-red, carmine, cerise, cherry-red, cherry, crimson, reddish, ruby-red, ruby, ruddy.

As nouns, scarlet is a hyponym of red; that is, scarlet is a word with a more specific, narrower meaning than red:

red: red color or pigment; the chromatic color resembling the hue of blood
scarlet: a variable color that is vivid red but sometimes with an orange tinge

red (noun)	scarlet (noun)
red color or pigment; the chromatic color resembling the hue of blood	a variable color that is vivid red but sometimes with an orange tinge
emotionally charged terms used to refer to extreme radicals or revolutionaries
the amount by which the cost of a business exceeds its revenue

red (adjective)	scarlet (adjective)
(especially of the face) reddened or suffused with or as if with blood from emotion or exertion	of a color at the end of the color spectrum (next to orange); resembling the color of blood or cherries or tomatoes or rubies
of a color at the end of the color spectrum (next to orange); resembling the color of blood or cherries or tomatoes or rubies
characterized by violence or bloodshed