Colours of average quality (8)
I believe the answer is:
standard
This clue is a triple definition!
'colours' is the first definition.
(I've seen this before)
'of' is a link.
'average' is the second definition.
(similar in meaning)
'quality' is the third definition.
(standard is a kind of quality)
(Other definitions for standard that I've seen before include "Flag - of the usual kind" , "Normal, average" , "Guide-line" , "Ceremonial flag" , "Degree of quality" .)