Rule 4 appears non-universal to me. Japanese historically developed ao (blue) before midori (green) which is still considered a shade of ao. I guess you could argue since the Japanese conception of ao includes both blues and greens that it fits, but it seems odd to me.

Aren't you imposing the modern meaning onto the past? The word didn't mean specifically blue or green originally, but rather both, right? I don't see how this would contradict the fourth rule.

Ao still doesn't specifically mean blue or green now.

Curious how this works in the modern world, where color is mostly globalized? If I ask for “blue” and I only want that what does it get translated to?

It gets translated to ao. Here's the Japanese wikipedia page ( https://ja.wikipedia.org/wiki/%E9%9D%92 ) which among other things notes that the word corresponds to "英語のblue".

But ao remains the word that refers to the color of plants. https://jisho.org/word/%E9%9D%92