Colour systems: RGB, CMYK, HSB

RGB is an additive system: red, green and blue light add up to give a colour, and the brightness of each runs from 0 to 255, so 256·256·256 = 16 777 216 colours result; it belongs to monitor, phone and television screens. Green and blue make cyan, red and blue make magenta, red and green make yellow, all three at full brightness make white, all zero make black; when R = G = B a grey results. CMYK is a subtractive system: inks absorb part of the light that white paper reflects; the primary colours are cyan, magenta and yellow, and black (blacK) is added because the three inks do not give a pure black in practice and black ink is cheaper; values are given in percent and the system is used in printers, plotters and publishing. HSB is handy for designers: Hue is the angle on the colour wheel: red 0°, yellow 60°, green 120°, cyan 180°, blue 240°, magenta 300°; at Saturation 0% a colour turns grey; at Brightness 0% it is black. Colours opposite each other on the wheel (red–cyan, green–magenta, blue–yellow) are complementary, and if one is taken for the background and the other for the text the writing stands out clearly. Red, orange and yellow are warm and blue and cyan are cool; the effect of colours on mood depends on culture and situation, so it is not a strict rule.