Blue, could be Black, White, Red, or Yellow (3)
I believe the answer is:
sea
I believe this clue is a quadruple definition!!
'blue' is the first definition.
(thesaurus)
'could' is a link.
'be black' is the second definition.
(as in the Black Sea)
'white' is the third definition.
The answer and definition can be both natural objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
'red or yellow' is the fourth definition.
(I've seen this in another clue)
Can you help me to learn more?
(Other definitions for sea that I've seen before include "All at . . . - totally confused" , "Ocean area" , "70% of the earth's surface" , "The deep" , "Large expanse of water" .)