Rosehip Mole asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
Blue green
Let's mix two colors, blue and green, and see what shows up depending on whether we're using light or paint.
With light — like on a screen — colors ADD together. Blue light plus green light makes cyan, a bright blue-green glow.
With paint or ink, colors SUBTRACT light instead. Blue paint plus green paint gives a duller, darker teal — not brighter, because pigments soak up more light the more you mix.
So the rule is: on a screen, blue and green light stack up brighter into cyan. On a canvas, blue and green pigment blend down darker into teal.
Same two colors, opposite outcomes — light gets brighter when it adds, paint gets darker when it blends.