It's blue, green, or maybe Red (3)
I believe the answer is:
sea
'it's blue' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sea that I've seen before include "The main" , "The tide" , "Body of saltwater" , "one may be pounding the beach" , "& 14 Ozone" .)