Why mixing paint and mixing light give opposite answers.
Red and green paint make a muddy brown. Red and green light make yellow. Both are correct — they're just two entirely different physical processes, and confusing them is the single most common color-theory mistake.
Additive mixing (light)
A screen starts from black — no light at all — and adds red, green, and blue light on top of each other. Add all three at full strength and you get white, because you're adding more light, not less.
This is why screens use red, green, and blue (RGB) as primaries instead of the red-yellow-blue most people learn for paint: human color vision has three types of cone cells, roughly tuned to those three wavelengths, so combining just those three lights can reproduce almost any color a screen needs to show.
Subtractive mixing (pigment, ink, paint)
Paint, ink, and dye don't emit light — they start from white light hitting the surface, and each pigment absorbs (subtracts) some wavelengths while reflecting the rest back to your eye. Mixing pigments means each one is removing more of the light, so mixing more colors together tends toward black, not white.
Printing uses cyan, magenta, and yellow (CMY, plus black ink for practical reasons — CMYK) as primaries instead of red-yellow-blue, because those three absorb light more predictably and produce a wider, more accurate range of printable colors.
Same wheel, two different physical processes
The color wheel itself is a model of human perception, not of physics — it works the same way whether you're mixing light or pigment. What changes is which direction "mixing more" moves you: toward white for light, toward black for pigment. Keep that in mind and the red+green paradox stops being confusing.