As far as I'm aware, only the prescriptions have to be identical. The question of tint is nihil ad rem. But I use a photochromatic pair as 'normal' and take a spair pair of dark-tinted ones by day or - if I have to go night flying - a spair pair of plain tinted ones then.
Incidentally, for those who may not be aware of the fact, varifocals are now OK if you have become 'accustomed' to wearing them.