Yellow food, or black, say (6)
I believe the answer is:
butter
'yellow food' is the definition.
'black say' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'say' becomes 'utter' (I've seen this before).
'b'+'utter'='BUTTER'
'or' acts as a link.
(Other definitions for butter that I've seen before include "Solid fat churned from milk" , "Edible fat" , "goat?" , "Milk product" , "& 22 Mincepie accompaniment" .)