WW2 Blue submariners jersey (the wearing of)

[fxstc]

The question pertains to WW2 RN officers aboard 'surface vessels' and not submarine personnel

We all know that officers in winter time could wear a white submariners jersey/roll-top - but - I have seen one image of an officer (Captain rank) also wearing the dark blue version as well under his working dress (battledress)

Was this common practice re. wearing the blue version? - why did they make a 'blue version' in the first place?

 

[fxstc]

Help anyone????

 

[Turnip]

That might have several reasons such as branch, first or second (work/battle dress) fiddle, availability …