The image above, when viewed at full size, contains one million pixels (a thousand by a thousand) and each one is a slightly different colour.
Each pixel in the display of your computer or phone contains three tiny lights: one red, one blue and one green. The display sets the apparent colour of a pixel by adjusting the intensities of those three lights – they are so small and close together that your eyes mix the three back into a single colour in your vision.
Inside the computer, a colour is represented by three numbers, representing the intensities of each of the red, green and blue lights on a scale from 0 to 256. (This is known as an RGB scheme for describing colours, often as distinct from the CMYK scheme – for cyan, magenta, yellow and black – that is used in printing.)
In the image above, the pixels methodically step through the gamut of RGB colours. The top left pixel is black – all colours set to zero – and the bottom right is white – all colours set to the maximum. In between, each small square works through the range of red and green values while the intensity of blue increases from square to square.