Hi AdvocatusD
As regards inside wheel arch paint I'm pretty sure it's black paint / underseal.
In the Lancia sales brochure for 1959 covering all Flaminia models
the inner wheel arches look black. Inner arches on my own 59/60 Flaminia PF
are also black.
Black and white images from contemporary Zagato road tests also look black
although if the car was dark it's more difficult to tell on a B/W image.