The RGB colour model adds the three primary colours, red, green and blue in various proportions to black (no light), and is consequently known as an additive model. An eight-bit, 256-step scale for each of the primary colours, used in this example, allows about 16.7 million different colours to be produced (256 x 256 x 256). Each of the three primary colours is allocated a value in the range 0 - 255 for each pixel, giving 24-bit colour. True green has a red value of 0, a green value of 255 and a blue value of 0. Yellow has a red value of 255, a green value of 255 and a blue value of 0. Orange might have values of 255 for red, 128 for green and 0 for blue. White results from a combination of all three primary colours.