Bright and Dark.
Like sadness and happiness, Memories to are of two types. The one which gives us a feeling of happiness when we remember it.
and the other one Which makes us sad!
Memories are the important part of our life and believe your life is empty with memories regardless of it is sad or happy.
This image is clearly reflecting both types of memories. The upper side is for bright and happy memories and the darker side is for dark and sad memories and that why you named it as