It is a real usage recognized by various dictionaries (Merriam-Webster, Oxford, et. al). Doing a search will find said usage in the definition listings, and "a riot of color" seems to the typical example given by such. If I had a copy of the Shorter Oxford to hand, I'd be able to say which usage came first, and if one derived from a figurative use of the other, or if they both evolved from a common, now obsolete definition. But I don't have a copy of the Shorter Oxford to hand, so we'll just have to be happy that the definition itself is attested by various dictionaries.
It's not quite the Old Oxford, but Merriam-Webster is easier to search online and seems to appear that it's a confusing mix of both. "Riot" is a Middle English borrowing from French, where it meant "public debauchery" (now considered an obsolete meaning), but even in French at the time had associations with violent debauchery. The first uses of the word in English were as a noun and for "debauchery". The first uses of riot as a verb were figuratively in the "riot of color" sense, but even then implied violence may have been part of the figurative imagery?