Trying to find an exact or "correct" buff (sometimes called yellow) colour is almost impossible. The paints were generally mixed on site as required - 50% of this, 20% of that, etc - and ships were repainted by captains, often at their own expense, in an effort to make their ships "showier" than their rivals.
There are, of course no original colour photos. Contemporary prints and painting vary considerably, even those done by the same artist - Fred Mitchell is a good example (See Ships of the Victorian Navy by Conrad Dixon)
Personally, I use Humbrol 154 - Matt Insignia Yellow, as I think this gives the right impression. Others may have different views, but I don't think there is a definitive answer.
Rick