It can be red, yellow or black but it's mostly blue (3)
I believe the answer is:
sea
This might be a double definition.
'it can be red yellow or black but it's mostly' is the first definition.
The definition suggests a singular noun which matches the answer.
'blue' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for sea that I've seen before include "The drink" , "that's easily reached from Dover?" , "Body of saltwater" , "Ocean area" , "North, say" .)