entre chien et loup (between dog and wolf)

I never knew my great-grandfather, and no one in my family ever did. The story passed down to us was that of a war hero who died from his wounds after World War I. This is who he is in this studio portrait hanging in my parents’ apartment.
On the other picture, found a few years ago in a local newspaper, he is the murderer who killed his tenant before taking his own life.

I have always been charmed by the French expression  “entre chien et loup”, which describes that moment of day when light becomes uncertain, at dusk, the final hours before night. In this in-between time, when the sky sometimes turns a deep blue before darkness takes over, it becomes difficult to distinguish shapes or beings. How can one tell a dog from a wolf?