The color of the substrate is being decided by how high the temperature it went up to while in fermentation. Think it like how much has it been burned. Technically, color of the substrate doesn't really matter with the nutrition level. Also, depending on the method of fermentation, color may just stay quite light as well. In case of smell, I would say it smells like regular soil. Everyone says different ways, usually described as "earthy" or "soil-like."