This pair of White-faced Storm-Petrel west of Porto Santo, Madeira showed significant differences in facial pattern and a grey upperparts.
Note the second birds broader, white, eyebrow, smaller ear patch, and overall paler appearance.

I have read that the juvenile has a greyer plumage, but there are also some variance in the adult plumages (races?).
One suggestion I've gotten is that a 2'nd calendar year bird could be involved here?

However feel free to send me a mail with your thoughts or experiences.
Especially from those who investigate museum collections and knows the sexes or ages of this species and its races