Drab is often used to describe how something looks, such as muted or muddy colors, while dull can be used to talk about whether something is sharp or not. As in "The landscape is so drab in winter." or "This knife is dull." You can also use dull figuratively about someone's wit or intelligence, though this is a little old-fashioned now. "My dog doesn't learn that well. He's a little dull."