The answer is all in the explanation. The soap dried onto the HOT black anodized surface. It literally cooked on it in the sun. The cooked soap, regardless of PH, is now a thin layer of product sitting on top of the colored metal which is refracting light and causing the rainbow effect. The OP's description of being able to remove it with additional water and soap indicates the product is removed. PH would cause damage and would not remove with soap and water.
I would not blame the soap, the PH or anything else outside of the technique of washing a car in hot sun and failing to remove the soap before it cooked on the surface of the metal.