Images are displayed on a computer monitor as bitmaps. A bitmap is a grid of pixels. The pixels may be represented using different numbers of bits which correspond to the number of colours present in the available range.

For example: If 1 bit is used to represent the colour of a pixel, it can have 2 values - black and white (colours may vary from monitor to monitor). This is properly described as a monochrome although monochrome is usually used to describe a grey scale picture.

#### Grey scale

Uses 8 bits to give 256 colours of grey, from black (0) to white (255)

#### Colour

Normally uses 24 bits to represent levels of red, green and blue: $\color{red}{\overset{11111111}{\mathsf{Red}}}\ \ \color{green}{\overset{11111111}{\mathsf{Green}}}\ \ \color{Blue}{\overset{11111111}{\mathsf{Blue}}}$ These values are often represented as decimal or hex values such as 255,0,255 or #FF00FF. 24 bit colour allows for up to $256\times256\times256$. ie $16,777,216$ different colours. The number of bits used to represent colours in bit maps are called the bit depth.

Note about printing Computer displays use (effectively) coloured lights to produce an image. 24-bit colour displays use RGB to represent as many colours as possible. Printers usually use 4 colours: Cyan, Magenta, Yellow and black. Accurate conversion from RGB to CMYK is difficult to achieve