According to this:
#FF0000#0000FF So what was the reasoning behind making green not equivalent to #00FF00 but instead #008000?
Although #00FF00 is a pure green in the way a monitor displays green light, #008000 is a more natural green, more like green things appear in nature and thus what people expect when they ask for something green.
If a person who wasn't familiar with the RGB color model said "I want the background to be green," I'll bet they'd be expecting something more like a leafy-green #008000 than the electric-lime-green #00FF00.