Black and white are described as colours by some photographers, while others disagree. The discussion is largely academic, but does have some relevance to the achievement of a basic understanding of colour.
The truth is dependent upon the medium in which they occur. In the case of light, black in not a colour. Black is the complete absence of electro-magnetic radiation having wavelengths within the visible spectrum. Go in to a photographic darkroom, close all the doors and switch off the lights. No photons can enter the room and there is complete darkness. White, on the other hand, certainly is a colour in the world of light. It is produced (as in the case of daylight) by mixing together all the wavelengths in the visible spectrum. Add together all the colours of the rainbow and the result is white light.
In the printing world, where colours are produced using pigments, both black and white may be considered as colours. In the case of black, the matter can be resolved by mixing together the remains of numerous tins of differently coloured paint. As more colours are added, so the resulting mix approaches black. White is a more difficult case to argue. It is not possible to mix pigments of other colours to produce white, so in that sense white is not a colour in the printing world. However, white pigments exist in nature, and are produced from substances such as chalk, so it is also possible to argue that white is a colour. The jury is still out on this final case.