Why is armor and weapon quality coded in this particular order: white > green > blue > purple > gold?

Is this a classic RPG convention that I'm not aware of? I saw the same colors in World of Warcraft and I expected ESO to change things a bit, ie swap blue for yellow, white for green etc.

It's pretty universal to be fair. Did wow create it? Not something that bothers me either way.

it was something diablo 1 had back in the 90s.

wow didnt invent much of anything

This, everything else would be a bit confusing, since ESO has enough differences to other games, they do not need to reinvent the wheel with some minor aspects...

I'd say it's a standard we're used to and feels familiar from back in the day. When I see the colors I don't need to think about the quality of the items, I just know it by heart, saves me a lot of trouble during looting, if I see white I don't bother checking the item, if I see blue, purple, gold I already know it's something that would interest me.

Not sure who was the first in doing this, but it makes perfect sense for MMOs to the use first as the standard. I can think of no reason to change it up.

It actually makes sense for elder scrolls though. The upper colors suggest elegance.

It was confusing for me at first because I was used to GW2's system (white, blue, green, yellow, orange, pink, purple).

But I guess that it is what it is.

Diablo 1 is the first time I remember seeing it. Before that the idea of rarity generally wasn't present as an item quality mechanic, only a descriptive term, and descriptively rare items weren't necessarily better than descriptively common ones.