An argument about colour (4)
I believe the answer is:
blue
'colour' is the definition.
(blue is a kind of colour)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for blue that I've seen before include "Colour; obscene" , "This movie is X-rated" , "Red-headed Australian" , "very cold" , "Varsity sportsman" .)