The term doesn't originate with computers, so the mono in monochrome doesn't refer to the 1 bit, but to the one colour. One dimensional colour isn't necessarily

The term doesn't originate with computers, so the mono in monochrome doesn't refer to the 1 bit, but to the one colour. One dimensional colour isn't necessarily grey so monochrome is only greyscale when referring to black and white images. If you make a cyanotype print of a black and white image the colours are blue, white and all the blue shades in between. One colour, but it's not greyscale.