1) Black and white lack hue and saturation, which are defining aspects of a colour. 2) We refer to colour-blindness as the inability to see colours. But colour-blind people can perceive black and white fine, therefore they cannot be colours. 3) No one, artist or layman, has ever referred to a monochrome picture or painting as 'colourful'

Well, I said technically speaking. Light with a frequency inside the visible spectrum is perceived by the human eye depending on its frequency spectrum distribution. In fact, several different frequency spectrum distributions might look identical to the human eye (so-called metamerism.) For example light that's a sharp spike at the frequency of yellow will look yellow to the human eye, but also light that has two spikes, one at red and another at green (and with proper amplitudes) will look exactly the same color to the human eye (even though the properties of those two spectra are completely different.) The reason for this is that our eyes do not perceive all frequencies within the visible light spectrum equally because we only have three types of light receptors. These receptors perceive slightly overlapping ranges of light approximately around the red, green and blue frequencies. There are colors perceived by the human eye that have no single frequency. For example there is no frequency that corresponds to magenta. Magenta is a "fake" color, an interpretation of our brain that's triggered when the light has red and blue frequencies. What happens when all three types of receptors are activated in an equal manner? Again, we get another "fake" color perception like magenta. In this case it's perceived as white. Technically speaking there's little difference between the two; in both cases more than one receptor is activated, and our brain interprets the result in a certain manner. In fact, the vast majority of colors in practice activate all three types of receptors in some manner. (Real-life surfaces and lights extremely seldom have pure single frequencies, or completely lack frequencies at the lower or higher ranges of the visible spectrum.) It's just that they activate some receptors more than others, which is why not all colors look white. In case of white, they are all activated almost in an equal amount. Technically there's little difference between white and other colors. It's just our subjective interpretation of white being somehow different from the others. (Black is truly a different matter. Pure black is the complete lack of any light, which truly can be considered a non-color.)