The various pillars of different materials and colors may well have been donated by different people or communities. The ‘riot of color’ being incidental to the statement of unity represented by the pillars.
Good suggestion.
According to the article, which, by the way, is exemplary for its obsession with the minutest details, the pillars were likely re-purposed from dilapidated Roman temples in the area felled by the earthquake of 363AD, shortly after the Constantine Edict made Christianity the official religion, and about a couple of hundred years before this cathedral was built.