A light source can be anything like sun or a bulb or a flash light. Basically a light source emits all frequencies within visible range of a human eye. The light coming out of these light sources is a white light. A white light is nothing but a combination if all frequencies. When this white light from a light source falls on an object some frequencies are absorbed and some are reflected back by the object. The combination of these reflected frequencies makes the color of the object. These frequencies which are reflected back by the object are known as the dominant frequencies.