It's simple why they're shades. Black is the absence of any colour or light and can exist without any light(colours are reflections of light). Meanwhile white is all colours(In nature, not things like paint or crayons). Some consider white a colour and not black since it technically uses light, unlike black. Meanwhile, grey is considered a 'neutral' colour since it lacks hue and has no place on the colour wheel(Example above. Technically, the colour wheel has many more colours, but a more simplified version is shown)