In the previous post comments there was an interesting debate on the etymology of the word riot. It's a French borrow in the Middle English era, so its etymology is at best "complicated". The "riot of color" idiom was one of the first written usages in English indeed predating the use of the term for violence, but the French word was "debate/quarrel" so it's hard to rule out that violence or negative connotations weren't always hand in hand in the metaphor underlying "riot of color".