For both, the vanilla game does not care what images you have or do not have, nor does it care about any of the extra tags you add to them. It will pick a random image which has the race in it's file path, including file name, it won't even attempt to make sure the sex matches.
The improved random portraits mod will attempt to use images that are tagged similar to the person, at least for the attributes that it tracks.
Yes, if you add white to the haircolor array for humans, then you will get humans with white hair. Though you should be aware that humans are used as a template for many other races, so those races will also get white hair as well.