Was there a reason why you used the ADY in dry form? That is a good method if you want the dough to last several days of cold fermentation, but when used dry it slows down the fermentation process and limits the byproducts of fermentation that contribute to crust flavor (and color, taste and texture).
As an aside, I did a bit of research on butter this morning. Although butter contains a small amount of milk solids (about 2%), it apparently is very low in whey content. So, there won't be much lactose to increase crust coloration. On the other hand, butter starts to break down at around 160 degrees F, and the milk solids will start to darken as they cook. Can you describe the crust color in a bit more detail, especially at the sides of the pan? I notice that the bottom of the crust seems darker than the sides. Was there a reason why you decided on a bake temperature of 555 degrees F?