How does the encounter with the "negroes" explore the theme of racism?
I will assume you are referring to the scene in Chapter 4 in which Nick is riding into town with Gatsby. At one point, Nick mentions that they pass by a car of African Americans.
...a limousine passed us driven by a white chauffeur, in which sat three modish negroes, two bucks and a girl. I laughed aloud as the yolks of their eyeballs rolled toward us in haughty rivalry.
This is an interesting passage because it is so brief, but the very fact that Fitzgerald chose to include it suggests that it is significant and that he is trying to say something about race. From this encounter, it is clear that there is some kind of competition between the two races. The blacks mentioned are no less refined than Nick and Gatsby. They are "modish" and are driven by a white chauffer. Clearly, they have money to pay a white person to drive them. The fact that they rolled their eyes with arrogance shows that in some way they feel superior to Nick and Gatsby.
It is interesting, then, that Nick laughs aloud at this exchange- if you could even call it an exchange. He is undermining the arrogance of the group in the car. It may be because he believes he is above them for the simple fact that he is white. However, he could just be laughing at the absurdness of competition in general. He knows that with Gatsby, he is setting himself for adventure, and it seems he is laughing at his own predicament, as the following line suggests:
"Anything can happen now that we've slid over this bridge," I thought; "anything at all."
Nick seems to be open for whatever Gatsby's company will bring. He is just along for the ride. The encounter with the negroes may highlight the racism of the time because of the separation and rivalry between the two races, but more than anything it shows Nick's sense of disbelief about what he is getting himself into with Gastby.