@hbg-gw-enthusiast I kinda do the opposite with Red as Heavy Battleships (and other advanced tech), after all the red ones go faster.
I would avoid using any colour which doesn’t sit with a nation’s colour scheme such as black.
So I use
White – Transports,
Grey – Torpedo Boat Destroyers, Coastal Submarines, Coastal Def Ships
Brown – Submarines
Purple – Destroyers
Dark Blue – Light Cruiser
Light Blue – Heavy Cruiser
Green – Battlecruiser
Yellow – Battleship
Orange - Fast Battleship
Red – Heavy Battleship, Adv. Submarines
For Carriers
Bronze - Escort
Silver - Fleet
Yellow Gold - Heavy