Each sensor contains two microphones which face inwards and detect the supersonic shockwave of a bullet as it passes through the target frame.
The base of each sensor is colored to indicate which corner of the target it should be placed. You can unscrew and rotate the base in order to re-assign a sensor to a different location.
The white sensor is the same as any colored sensor (once you rotate the base) except that it will be less confusing if you need to replace one sensor and you don't have the correct color.
Remember, if a sensor is shot, you'll probably also need to replace the bracket.
If you need all four colors, order the 4-pack.