Mud & Glow - Why does mixing paints make brown but mixing lights make white?

Mixing paints makes brown because every colour you add absorbs more light, while mixing red, green and blue light makes white because light adds up. Swirl six wet paints on a theatre backdrop until the whole picture collapses into mud, then drag three spotlights together until the stage blooms pure white - and save the show with the light you have left.