[Jimbuna]

Originally Posted by Reinhard Hardegen

The uniform for an enlisted sailor consisted of a jacket, a pair of trousers, a white and a blue shirt, matching collars edged with three stripes, a silk neckerchief, grey gloves and a cap with two ribbons. An officer wore a midnight-blue [1] double-breasted jacket with ten brass buttons and a matching peaked cap. U-boat personnel also wore jackets and overtrousers of brown or grey leather. As an unwritten rule, the captain of a U-boat wore a white peaked cap.

When U-boats were at sea, there were few dress restrictions. Full uniforms were typically worn on departure from and return to base, but due to the cramped and humid conditions, U-boat crews wore more comfortable civilian clothing on patrol. These included seaman's jumpers and sleeveless shirts. Lookouts wore oilskins and sou'westers on duty. A grey-brown denim "battle-dress" uniform was also worn on patrol, the original issue being from British uniform stocks abandoned at Dunkirk.

So from what I have been reading it was to quickly identify the commander in times of need, as if the lights go out from damage, you can see the white cap better then the blue in close quarters. Which is why only the boat commander wore it, unlike the other ships within the navy.

Hope that helps.

Rgr that.