Not a bold colour (6)
I believe the answer is:
yellow
'colour' is the definition.
(I know that yellow is an colour)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for yellow that I've seen before include "Offensively sensational" , "Primary colour" , "centre of egg" , "Custardy shade" , "(Of paper) age" .)