Word pairs consisting of bright/light vs dark/heavy usually only differ in their "feel", but sometimes, each might develop its own special meaning. 덤덤 and 담담 seem to be one of such cases. 덤덤하다 describes displaying little feeling or emotion when it is expected, like a person showing no reaction on winning a prize or meeting a family member after a long separation. 무덤덤하다 is a stronger version of this sense. 담담하다 typically describes a calm and collected state of mind, behavior, and other things, implying peace and composure. So 담담하다 usually carries a more positive nuance than 덤덤하다 or 무덤덤하다 which often implies dull and boring. Both words also describe food that is very bland in taste. - 친구는 경주에서 1등을 하고서도 (무)덤덤한 표정이었다. - 시험을 잘 치른 영희는 담담한 마음으로 결과를 기다리고 있다. - 찌개 맛이 너무 덤덤하다/담담하다.

Hi. Both words are almost same meaning. Only different is scale of feeling. "ㅏ"is more light feeling than "ㅓ" for example, 빡빡하다 is more light feeling than 뻑뻑하다. 살랑살랑 is more light than 설렁설렁. Thank you.