Some time ago I tried to apply the red cross on a ship's hull, but it didn't look satisfying to me. That's due to two reasons:
1) SH5 uses the texture files for the ship's paintwork by adding the texture's colour to the base colour of the model file. So it's quite difficult to get an opaque colour.
2) The texture files have to be huge because you need a very high resolution. Otherwise the red cross image on the vessel's side will look blurry.
Therefore I decided to stay with the Harbour Flags Mod. It already contains a flag of the International Red Cross in the flag.dat and an entry for the IRC in the defside.cfg. So you're able to create your own hospital ships in \Roster\RedCross\ and corresponding traffic that you may wish to include via MissionEditor2 in the standard campaign. I wish I'd have more time to add all this stuff to SH5...
