A red filter (usually you would use a light red for portraits, if at all) can lighten skin blemishes for portraits, but it can also make the lips unnaturally light. In the old days they used to use green lipstick to make the lips darker.
The density units .3, .6, etc. are logarithmic units (LogD), and they are the same units that are used on a transmission densitometer for measuring density on a negative. So when Adams suggests that exposure should be adjusted so that Zone I density is .1 and for typical enlargements development time should be adjusted so that Zone VIII density is around 1.2 (the actual value will depend on the paper, enlarger, and print developer, but 1.2 is not a bad starting point), these are the same units as the units used to describe ND filters.