Why does the printer use the three secondary colours: cyan, magenta and yellow?
Yellow ink appears yellow because when white light (W = R+G+B) is shining on it, it absorbs the blue and reflects red and green light which we see as yellow (Y = W-B). Cyan ink absorbs red and reflects blue and green which we see as cyan (C = W-R). Therefore, if we mix yellow and cyan inks then both red and blue light are absorbed by the mixture, and green is reflected – it will appear green: Y+C = W-B + W-R = W-B-R = G (green). Any combination of the three secondary colours (CMY) will produce one of the primary colours (RGB). Varying the amount of each coloured ink will produce all the colours possible.