Strictly speaking, neither 'black' nor 'white' are colours. Black is the total absence of all light (of whatever frequencies). White is the full set of all frequencies (at their usual amplitudes) found in daylight. (i.e. it's the combinaton of lots of colours).
However, since there is some light there, it makes some sort of sense to refer to 'white' as a colour but 'black' can never (scientifically) be regarded as a colour.
So a monochrome display has only ONE 'colour present (i.e. 'white', which is actually a combination of lots of colours). It also has areas where there is NO colour present (which we refer to as 'black', but it's not a colour). Thus 'monochrome' makes sense!
;-)