system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why are the primary colors for light (green, red, or blue) different from the primary colors for paint (yellow, magenta, or cyan)?
gpt
The quick answer: mixing light is "additive" and mixing pigment is "subtractive". But that's essentially just restating your thread title in different terms.

With pigment, assume that the object is being illuminated with all frequencies of visible light. The pigment will absorb some frequencies and reflect others. The reflected frequencies reach our eyes and that is the color we perceive. So pigment subtracts frequencies from the visible light spectrum. If we add enough different pigments, we subtract all the frequencies and are left with "black".

With light, we start with the assumption that there is no visible light emittance (black). Adding frequencies causes the eyes to perceive a color. The more frequencies we add, the closer we get to the full spectrum of visible light - "white".

Essentially, when mixing pigments fewer frequencies reach the eyes, but when mixing light, more frequencies reach the eyes.

So why is green a primary color in light? Consider secondary colors and color compliments. A primary color mixes with it's compliment to produce white light. Red and blue mix additively to create magenta. Magenta and green mix additively to produce white.

Edit: What's important to remember here is that primary and secondary colors aren't a physical property of light. Color theory is based on the physiology of the eye. Humans have three types of cone cells in the eyes which respond to three different ranges of colors.