AThe yellow paint can was left over from the painter who painted the walls in the interrogation room where Harry was being questioned. In several other scenes that show Harry's boots, e.g., in the cemetery and the interrogation room, it can be seen that they are splattered with yellow paint, i.e., Harry was the painter of the walls. It's to show the connection between him and his surroundings. Before his arrest, he spent time there as a maintenance person to become familiar with the floor plans so he could later escape.