The crackers are baked in a moderate oven (350 °F) for approximately 20 minutes. It takes that long for the colour to develop in them because remember sourdough discard tends to be quite acidic! Sourdough starter has a pH below 5, so the dough tends to be quite acidic too and Maillard browning reactions will be slower. Maillard browning is faster at higher pH (more basic or alkaline).