Graduate engages someone to promote book: The Color Purple (7)
I believe the answer is:
magenta
'purple' is the definition.
(I have seen 'Reddish purple' mean 'magenta' so perhaps 'purple' could also mean 'magenta')
'graduate engages someone' is the wordplay.
'graduate' becomes 'MA' (abbreviation for Master of Arts).
'engages' indicates putting letters inside.
'someone' becomes 'agent' (agent is a kind of someone).
'ma' enclosing 'agent' is 'MAGENTA'.
'to promote book the color' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for magenta that I've seen before include "Bright purplish-red" , "Shade of purple" , "Light purplish-red" , "puce" , "Purplish-pink colour" .)