An underwater hull can require a different paint mixture from the exposed topsides. The immersed surface cannot be repainted as often, so its coating has to remain in service longer. A consistent underwater colour also avoids a conspicuous paint line at the waterline. Grey is therefore a plausible treatment even when a model builder expects another colour.
Notable Point · 3:17:51-3:18:23
Why underwater hull paint can differ from the topsides
Different repainting cycles and waterline requirements can justify a separate coating and colour below a warship's waterline.