The Man in White

I asked this question several years ago and never received a verifiable answer.

At about the thirty minute mark, the ship’s crew is singing The Royal Navy Hymn (Eternal Father, Strong to Save).

After Captain “D” says “Let us pray”, there is a shot of the crew.

Everyone is in blue save one man who is dressed entirely in white.
A single breasted jacket and under his arm is a Royal Navy frame cap.
The ratings on his arm is a fouled anchor with one chevron below it.

My question remains:

Why would one crewman be in white while all the others in blue?

I’m sure this was done on purpose.
What would be the duties of a man whose uniform stands out from everyone else?