The German fire service was absorbed into the national police in 1939, and from that time onwards all fire trucks were painted police green and fitted with blue lights - which were already being used by police agencies. These were regular forward facing lights like this example at the time.
This took a while, but I expect by 1942 this pump was probably originally built in green with these lights, then refinished in red after WWII when fire brigades were returned to local authorities.
The rotating "lighthouse" type beacon lights came later, and became the norm in the 1950s, although not all appliances were switched over to beacons. I first visited Germany in 1958 and saw both types in use back then.
When the fire service was placed under police control, the national police was led by a certain Heinrich Himmler - so, among other things, he was responsible for the idea of fitting blue lights on fire appliances. Thanks.
Added by petros on 21 February 2021.