Can I use a primer as a models main color?

Hi;

     When I build my Tankers, That's what I usually do. Red Oxide on the Bottom and Decks and Black on the hulls and Tamiya White primer on the deckhouses. Then Clear coat everything for weathering and detailing.

I use red oxide primer for bottoms of all my freshwater models.  Some GL ships use red oxie all the way up to deck, so the primer works up to deck.  Some of these ships even use it on decks and hatches,so I do too.

Many airliner modelers use Tamiya's Fine White Primer as a basic color for predominantly white color schemes.

Hi,

I also use primer as regular paint on my models too.

I use stynylrez black primer quite a bit as a color actually. Depending what primer you're using and what weathering products used, you might just want to put down a quick seal coat. Stynlrez is a sealer too though so I don't bother. I use it for rubber hoses and belts and such and weather the tone I want right over the primer using water clean up oil stains. Or a craft paint wash for that matter. My engine compartment and engines and such are generally primed in black anyway to take aluminum or other metal paints and such.. So ya, have fun !

Most definately.   Rustoleum Red Primer is my go to for ship hulls.    Vallejo Model Air Gray Primer is often called out for US Ghost Gray.

Yes, I have done this when the primer color is right for the model.  Usually do white, for aircraft undersides.