A rambling Bakster build? You don't say!
The use of "public airwaves" is sore complicated. And government rgulations tend to be followed to the letter, at least until it causes more work for the regulators.
So, the code-key requirements remained. Were driver's licences done the same way, we would all have had to demostrate how to hitch horses to a wagon, and drive a stick shift. Much as the Master's Certificate at sea ocer required the ability to name all the sails of a fully-rigged ship.
Things change. Used to be, you needed to pass the VHF radio test to have a ship-to-shore "mobile" communications device for our automobiles. These exploited a "loophole" in the Radio Operators regs. Forty years' later we jst walk in the T-mobile store . . .
(Trivia: "T Mobile" was begun as "Trucker's Mobile" as an alternative to the overloaded 11m CB band for communications on the road.)
One of the complications of getting an (aviation) Pilot's license is needing to get the VHF/UHF radio licensing to operate the radio in the plane. (A Marine Pilot needs a Master's Certificate, a certain number of years on the given water way, and to pass a detailed test; and needs periodic recertification.)
Back to topic
From memory Verne is under-specific about just what Nautilus is made of. I believe it probably would have been iron, as that was the material of the age. That almost suggests a rail-brown sort of color. A chestnut roan sort of deep color that would be complicated (in an interesting sort of way to model) to render.
Now, the folk of that age were not ignorant cavemen, they new iron would rust. So, they might have "iron blacked" their submarine. Which was lampblack and coaloil in a lacquer base.
Thus, it could be a glossy paint sort of finish. And, of course, gilding would have been a thing.
I want to remember reading about another build, where there was discussion about going "full Verne" and suggesting a paint job that replicated a fish. Which would be suitable Victorian. Painting a submarine to resemble a Trout or Salmon would be totally in keeping for the sensibilities of the age.
Imagine an iron trout a couple of hundred feet long chasing one's ship.