Although Adah seems like the darker twin, I don't think you can honestly label her like that. Both Adah and Leah change. Adah is pessimistic, but she's def not evil. Adah is extremely misunderstood. She does not conform to society's standards, and, as a consequence, is seen as a threat. Adah really cares for the the Congolese. She sees her old lifestyle as extremely lavish. (The brands of toothpaste example) She values all life--later on in the book when she quits her job as a doctor because she cannot stand the fact that people's lives in the US are being saved when there are so many deaths in Africa.
Leah actually becomes darker. When her faith in her dad is shaken, she loses faith in everything else except her love, Anatole. She gives up the idea of religion even. So, in this sense, A&L switch places.