mullet vs red mullet

mullet and red mullet both are nouns.

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

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

mullet: bottom dwelling marine warm water fishes with two barbels on the chin
red mullet: brightly colored tropical fishes with chin barbels

Other hyponyms of mullet include goatfish, Mullus surmuletus, surmullet.

mullet (noun)	red mullet (noun)
bottom dwelling marine warm water fishes with two barbels on the chin	brightly colored tropical fishes with chin barbels
freshwater or coastal food fishes a spindle-shaped body; found worldwide
highly valued lean flesh of marine or freshwater mullet