The two most telling examples are Candy's use of n- and the description of the boss giving Crooks hell whenever he's mad about something else.
Throughout the novel, Crooks is regarded as inferior by all of our characters, save only Lenny. This inferiority peaks with Curley's wife and her reminder of how quickly Crooks could be strung up.
Candy does experience a change of heart in that same chapter. We see him defending Crooks against Curley's wife. This suggests that Candy's form of segregation was probably societal instead of personal. He hadn't ever entered Crooks's room because nobody else ever had.