Witnesses should never be questioned in front of a murder suspect.
When the soda fountain worker give the customer a fountain drink, the seltzer from the spigot comes out brown. Seltzer is clear. In 1950s all fountain drinks, including Coca Cola, required a squirt of flavored syrup then the cup was held under the fountain spigot. The clerk would push the handle back for a pressurized squirt to mix the clear seltzer and syrup then would pull the handle forward to fill the glass.