With respect to Bill/SFNM ,we can see from the recent photos of the Pizza he produced in a wood fired oven that this phenomenon is surely created by some other reaction.
The crust color of my last pizza, which was the first to use natural starter only, was different from my previous attempts. The bottom did have many small black spots, but the top just had a few larger ones and the edge was paler than normal. It should be noted that pizzas near the end of the batch had more spots on the top that earlier ones, so underproofing may be a factor.
My oven roof is more of a dome than the flattened shape Marco recommends that radiates more heat back onto the top of the pie. When removing the pie from the oven, I usually lift it up close to the roof for 10-15 seconds to give the top a quick, hot, blast. This time I did not do that because I wanted to observe how my oven handles the different natural starters. I've got another pizza baking session scheduled for tomorrow using the Camodoli culture and I'll try a few different things to see what the effect is on the color.
To be honest, I'm so thrilled to be using these natural starters and getting really good results, that color is less important than texture and flavor, but crust color can be a strong indicator of flavor. As the Japanese saying goes, "you begin eating with your eyes" .