Explain title of the novel White Teeth. Where does Smith explicitly reference the title in the novel and what does the scene demonstrate?

The title of White Teeth is instructive because it encapsulates one of the book's main themes—how society reinforces the harmful notion that our shared humanity is challenged and undermined by differences of race and culture. However, all of us, whatever our skin color, nationality, or racial heritage, have the same white teeth. It is the color of our teeth that identifies us all as being part of the same human family.
Paradoxically, white teeth can also symbolize racial differences. The children discover this in chapter 7 when they bring food from their school's Harvest Festival to an old man by the name of Mr. Hamilton. The old war veteran is an unabashed racist, and tells the children about the time he served with the British Army in the Congo. At that time, white teeth served to accentuate racial distinctions:

Clean white teeth are not always wise, now are they? Par exemplum: when I was in the Congo, the only way I could identify the n****r was by the whiteness of his teeth, if you see what I mean. Horrid business. Dark as buggery, it was. And they died because of it, you see? Poor bastards (p.113).

Ironically, Mr. Hamilton only has false teeth. This can be interpreted as further evidence of his general lack of humanity.