Grey is boring (4)
I believe the answer is:
dull
I believe this clue is a double definition.
'grey' is the first definition.
(thesaurus)
'boring' is the second definition.
('dull' can be similar in meaning to 'boring')
'is' acts as a link.
(Other definitions for dull that I've seen before include "Thick" , "Not bright or shiny" , "Lacklustre" , "Lacking interest" , "Not shiny; uninteresting" .)