It's Not Simply Black Or White
Achromatic colours (white, grey and black) have lightness but no hue or saturation. A chromatic colour is a colour which has even just slightest amount of hue.
A monochromatic colour scheme, means that designers use varying shades of only one colour, be they chromatic or achromatic .... although some will argue that achromatic colours can't be in a monochromatic colour scheme, even though the full achromatic range is indeed a single monochromatic scheme of white (a combination of all colour) ... this is an endless debate but something to be aware of.
