Are black and white colours? I am an artist, and I have heard that white is a 'tint' and black is a 'shade' in the artist world. I have also heard that they are colours in the normal world/scientific world. Which is correct and why?
 A: Light is made up of particles called photons. Each photon has its own wavelength, which the eye perceives as color. White light is a mixture of lots of photons with different wavelengths which the eye perceives as "white".
Black is the absence of light - sort of. When we color an object black and shine a light on it, almost no light comes back - it looks "black". But something crazy happens when we heat up this black object: it starts to glow... First a deep red, then bright orange, and finally white (hot). Yes, black can look white, under the right circumstances. A white hot piece of coal is a black body, as far as physics is concerned. 
The black object does give off "light", as it turns out - but instead of returning the light you shine on it (like a white surface or mirror), it sends out photons based in its internal energy. It is a bit like having a trampoline with lots of people on it. The more wildly they jump (hotter the object) the more likely some people will fall off (photon emitted) That is a black body. If someone gets on the trampoline (shine light onto it) they disappear in the crowd- the ones that fall off depend on how hard everyone is jumping, not that someone else joined. So photons are not reflected (turned around and sent back) by a black object. The mixture of wavelengths of photons emitted is different in composition (color) than the mixture sent in.
By contrast you can think of the white surface as a trampoline with a guard fence. New people (photons) get turned around and sent away. All photons are reflected, so the mixture of colors is the same as before.
This doesn't directly answer your question - because the artistic perception if color is only remotely connected to the physical interpretation (and explanation), there is no "absolute" answer to give. I hope the above begins to explain why.
One more thing: the perception of color comes from the fact that we have three types of color sensitive cones on the eye. Photons with different wavelengths will excite each of the three to a different extent so the eye might tell the brain "this light is 5% red, 20% green and 75% blue", depending on what mix of photons it saw. The brain turns that into "color". Until that last step, you can really only talk about mixtures of photons and their wavelengths. And once you are in the brain, you are dealing with perception, neurology, art, ... And you can call it what you like.
See for example this sensitivity curve (from http://www.colorcube.com/articles/basics/1sscurve.gif)

You can imagine that the same response (how much red, green, blue the eye picks up) could be achieved with different mixtures of wavelengths. This is the reason that sometimes two objects that looks like they have the same color will end up looking different when they are illuminated with a different light source (tungsten vs fluorescent vs daylight).
Color and color perception are quite a complex subject - but the bottom line is that for the purpose of your question, you can call it whatever you like. This includes saying that black (or your approximation of black) is just a "dark white", or a shade of white (gray) - because most of the time, the black you can make is not "really" black. When physicists try to do a careful experiment in black body radiation, they will cover the inside of a sphere with black soot, and treat a small opening in the sphere as an approximation to "black" - light going into the hole is very unlikely to come back out, but photons generated by the black surface has a chance to escape). You can't easily do that with paint.
