White plastic is highly reflective, making AirPods stand out against skin and hair.

Human vision is highly sensitive to contrast. White objects reflect almost all visible light, while human skin and hair are darker and absorb more light. This creates a high visual contrast. By putting bright white stems in your ears, Apple ensures that AirPods pop out visually on almost any person. If AirPods were black, they would blend into the shadows of the ear and hair, making them invisible from a distance. The white color ensures that other people notice them instantly, creating a powerful form of free advertising.