This probably means that the camera has switched over to night vision mode.
Each camera has a sensor that detects light, once this light source drops to a certain level the camera will flick over to night vision mode and become black and white. This is to ensure that the camera can provide the most optimum image quality in the dark.
Sometimes, only one camera may switch onto night vision mode at a time. For example, if you have a 4 camera system and one of the cameras is located in a shaded area, this will become black and white first, whilst the other still remain in colour. After a short period of time however, the remaining colours will also turn black and white so that night vision can see in the dark.