Is "brownout" a common or standard term in the industry? First time I see it.
Yes. Going from green to red is called “browning out”.
Is that the origin?
I thought it was an analogy to the electrical problem: flickering lights due to high demand.
Don't know the origin, but with no technical background past using Linux, I only ever heard of brownouts in contexts of failing (often 3rd world) electrical infrastructure. Mostly Africa and South America (don't mean to offend anybody living there, I know they're vast continents with many rich/infrastructure-stable countries too).