Q. Define Colour Depth in graphic display system?

It is clear that an image contains an array of pixels.  If we tell which pixels are 'off' and which are 'on' to the monitor, it must be able to display the image as a pure white and black image. However what about Colour and Contrast?  Obviously if just a single bit is assigned to a pixel we can't give any extra quality to image. It will look like a black and white line drawing. Such a system is characteristically known as a two-colour system.  Such black and white picture can be converted to gray levels by assigning more bits for example with two bits we can get subsequent levels:  White, Light Gray, Dark Gray and Black.

To add colour to an image we have to store colour of pixel with each pixel. This is typically stored as intensity measures of primary light colours - Red, Green and Blue.  Which means we have to assign more than 1 bit to describe a pixel? Therefore 1 bit per pixel means 2 colours or 2 gray-levels, 2 bits per pixel 4 colours or 4 gray-levels and so n bits per pixel implies a display of colours or gray-levels is possible.