Well, General Idea is that "You should be part of a Faction to be white with Guard", means if you want Neutral Liberty Navy Guard, you should do missions to get Navy IFF, then you'll be able to enter Guard System and so on.
Although for the factions that offer Multi Bribes, a Double Bribe would get you white with Guard. (Well, which gives you the faction IFF, still same thing as above)
Most factions need Total of 30 missions for Guard IFF, but Military Factions usually need 50 or so.