I use DME when making pretzels. However, that isn't what makes them dark. Boiling at a high pH, traditionally done with lye, causes changes to the starch molecules that deepen the Maillard reactions when they bake. You can get reasonable results with baking soda, but I recently heard of a technique where if you bake your baking soda first, you intensify the results during the boil. I don't have a link, but you should be able to Google it. I think it was from an article by Mark Bittman.