visible light - Is a white object always white?

Is a object that’s white with the lights on, still white although it looks different under a different light or in total darkness?

Answer

What makes an object white is its property to re-emit and scatter nearly all the frequencies (colours). During scattering, the directional information of the light that illuminated the object also changes; that is, the incoming light is scattered into a distribution of angles. To answer your question, an object's properties do not change if it is being illuminated by a different frequency or even if it is not illuminated at all.

While we are on the subject it is worth noting the difference between reflection and scattering. Unlike a scattering object, a reflective object preserves the incident angle. So a mirror is "white" in the sense that it re-emits nearly all the frequencies, but different from traditional white objects because it has definite scattering angles.