As an alternative to the black silk top hat, the grey top hat has become more and more popular. Some would consider the grey top hat a little less formal, which is why the black one would be preferable for weddings.

Grey Top Hats At Royal Ascot
Otherwise, they can be considered equal on the formality scale. Maybe this is due to the fact that these top hats are usually matte and made out of felt rather than silk plush. To my knowledge, Prince Philip is currently the only man who wears a grey top hat in silk plush. As you can imagine, these are an absolute rarity, even beyond the black version.
Interested in learning more about grey top hats? Get the full 263-page Morning Dress Guide here.