How old is your hull. Over time, the gelcoat bleaches out and becomes whiter ( but not white white)
The Spectrum color probably will be too dark. In those cases where I am more concerned about a bare spot with the fiberglass showing and not so much the color, I use Marinetex paste white . It is less expensive than Spectrum. Although I haven't seen theses in the store, there are MarineTex coloring agents that can be mixed into the white paste. MarineTex is epoxy based so I don't think that polyester resin gel coat dyes are compatible but that would be an interesting experiment.