Well the primer color basically is your personal choice and depends on what you are trying to achieve.
Since I‘m also quite a the beginning of my painting journey you might want to take a grain/bag of salt for the following:
You can use a color that you plan on using for a large part on your model and remove the extra steps to prime first and afterwards do a couple layers of base color (like deathguard Green primer instead of black primer and multiple layers of brush on DG green)
Grey’s and off whites derm. to work great for contrast paints.
Black is often used in combination with lighter sprays as a base for cenitals.
Personally I prime Matt black since it is very easy to see if there is enough primer on the model and therefore I can achieve a thinner paint job without clogging details. Also blacks have a tendency to cover a lot better than others like white.
The downside of this is that I tend to need more layers of base paint to fully cover the black primer.
I tried grey primer, but that is quite hard to see (on grey plastic) and therefore I tend to apply it way too thick (but it works great on colored plastic like in the conquest magazines).
As for your question about the bubbles, I had that happen in the following scenarios:
1. primer was applied too thick
2. primed from to close distance
3. wasn‘t moving the spray fast enough (too long on a single spot of the model, this also causes case 1)
4. very high humidity while priming
5. didn‘t thoroughly shake the spray
In your case it looks to me like the primer was applied quite thick.
ValerousHeart already put it in quite good words: you basically just want to dust your model with the primer, it does not need to be a solid coat.
That being said... I personally like having a solid coat of the primer since it feels like covering that with base colors is easier to get a spotless coverage. But this is purely personal preference.
I hope there is some useful information for you in this wall of text, also if somebody knows better, feel free to let me know since I‘d like to learn myself