What first gives light in the tanker and why does it matter?

Baba and Amir made their escape from Kabul in an old, tarpaulin-covered Russian truck. They survived an altercation with a Russian guard who threatened to rape one woman and, later, to kill Baba. After spending the night in a dark, dank basement in Jalalabad, they discovered that the second truck in which they hoped to escape had broken down. The chose another option--riding in the empty tank of a fuel truck. It proved to be a terrible ride to Peshawar, travelling in total darkness and breathing the fumes of gasoline. Amir was happy to at last see a small sliver of light: It came from the fluorescent green hands of Baba's wristwatch. Amir kept his eyes on the watch, which helped him to take his mind off their awful journey.