Part of Roy G. Biv (6)
I believe the answer is:
indigo
'part of roy g biv' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for indigo that I've seen before include "supplier of blue stuff" , "Sixth rainbow colour" , "Blue-purple colour in spectrum" , "Dark blue dye (plant)" , "Blue dyestuff" .)