I have never seen any first-hand account (or original colour-photo) in regards to colour used on the Seehund (or any other german WW2 midget submarines), so - as mentioned before - the input you get here are just more or less qualified guessing/suggestions.
The colourschemes found in modelmakers instructions are for sure just fantasy or based on the colour being used on the vessels displayed in museums (which were used by the manufacturer in the prosess of making the model kit). The museums are just using the paint they have at hand, so no help there. I think the only valid exception might be the IWM Biber, provided they stayed true to the original colour during the repaints being done since 1945 (it was captured during the war and kept in it's original state until being transferred to IWM).
All in all, there are no sure answer unless someone dig up original german regulations with name or RAL/RLM-code for the colours beeing used (I doubt it existed in 44/45, and especially that any such documents might have survivied in museums/archives). Ie: You'd be just as correct by making your own choice of grey/blue shade.
I have collected some photos/drawings in a zip-file for you, just in case you can find it useful in regards to the perioscope:
https://www.dropbox.com/s/tbk0z93z3hojd8s/Seehund.zip?dl=0