The more bits per pixel, the higher color variety and quality of the monitor. The first graphics cards and monitors supported color, which was monochrome.
Typical working colour schemes. Bit Depth, Number of Colours. 1 bit, 2 colours (usually black and white). 2 bits, 4 colours. 4 bits, 16 colours. 8 bit greyscale, 256.
In the black and white image, only two colours are needed. This means it has a colour depth of 1 bit. A colour depth would allow four different values: 00, 01.