Colour depth is the number of bits used to represent each pixel.

The idea

Colour depth counts how many bits are stored for each individual pixel, so it fixes how many distinct colours that pixel can take. With n bits per pixel there are 2ⁿ possible bit patterns, so colour depth n gives 2ⁿ colours: 1 bit gives 2 colours, 3 bits give 8, 8 bits give 256. A 4-bit pixel might store 1011, one of 16 patterns. More bits per pixel means more colours and smoother gradients, but each pixel needs more storage, so file size grows. Colour depth is a per-pixel property; it is not the total number of colours in the whole image and not the image's width or height. To answer, identify the bits per pixel and apply 2ⁿ.

What earns the marks

• Colour depth is the number of bits used to store each pixel.
• The number of colours available is 2ⁿ where n is the colour depth in bits.
• 1 bit gives 2 colours; 8 bits give 256 colours.
• Increasing colour depth increases the number of colours but also increases file size.
• Colour depth is a per-pixel value, not the image's pixel dimensions.

Common mistakes and how to put them right

• Confusing colour depth with image resolution.

  Put it right: Colour depth is bits per pixel, whereas resolution is the number of pixels (width × height).

• Thinking 8-bit colour depth gives 8 colours.

  Put it right: It gives 2⁸ = 256 colours.

• Treating colour depth as the total number of colours in the image.

  Put it right: It is the bits stored for each pixel, from which the colour count is calculated.

Exam tips

• Read the question for 'bits per pixel' and apply 2ⁿ before choosing an option.
• Check whether the question asks for bits per pixel or number of colours; they are different quantities.
• Link extra colour depth to larger file size when a question asks about consequences.