I mostly just prime the base with the rest of the model and I typically prime things white or Vallejo's light grey. I haven't actually owned a black primer in the past year or so because I ran out but was using it so infrequently I never got around to buying more.
If I do feel like priming bases separately from the model it's typically just with white primer because when I prime things I tend to just load up the airbrush and prime everything within arms reach

Don't want to have to clean it out and load a different coloured primer. I hate spraying primer, so I just try and get as much out of the way in one hit as I can,
lol.