Germans painted their propeller hubs in a spiral pattern for the same reason the Allies painted the tips of each propeller blade yellow - in order for crewmen on the ground to know if the propeller was spinning so they don't walk into a propeller in full revolution.

The blades are also painted in a black that almost completely eliminates reflective light, which is where the hazard for ground crews stems from.

Just to be precise, not all Fw 190s had a spiral on the hub. The Imperial War Museum's Fw190 A-8 has the hub split in two colors. The Fw190 A5 of Jagdgeschwader 54 that was shot down and later restored and on display at the Flying Heritage Collection has a solid hub and many other's from the same squadron had a solid cowling as well.