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The mixing behavior of coloured lights and coloured matter is different because in case of lights a mixing increases the amount of photons (= additive mixing). In case of colour matter the matter only reflects light according to the refrangebility of its surface and absobes the rest. In consequence coloured matter reemits less photons than it was exposed. So mixing colour matter leads to mixture that absobes more and more photons and becomes therefor darker (= subtractive mixing).