The prejudice with which Crooks is treated by many of the people on the ranch suggests that though men like George and Candy accept the cultural biases of their day, they do not actually generate these biases, nor do they consider they biases as grounds for moral superiority.
Crooks plays horseshoes with the men on the ranch and, though generally isolated, is not completely cut-off socially. He is expected to sleep in a designated place separate from the white ranch hands.
Crooks, the old black man on the ranch, lives alone, ostracized by the ranch hands because of his race.
There is a distance which George and Candy feel they should keep from Crooks which can be read as a respectful distance, allowing Crooks to bear his social burden with the dignity of solitude. The idea that Crooks should be allowed his own space is repeated by both George and Candy.
The fact that this support of Crooks' isolation is not accompanied by any disdain or claim of superiority seems to suggest that the racial prejudices of the era are not understood by George and Candy as moral precepts, but rather as practical and fixed rules of society.
Curley's wife, contrastingly, uses the prevailing prejudices to her advantage, threatening Crooks by essentially suggesting that because she is white and he is not, she can make whatever claim she wants and she will be believed. She has the power, then, to have Crooks severely punished by lying about him.
Considering these ideas, we might say that Of Mice and Men presents prejudice as a rather thoroughly accepted racial precept that does not necessarily translate into bigotry, though it can. The prejudice of the day can just as easily be understood as a regrettable and accepted set of boundaries, akin to those separating rich from poor.