My dad has painted 3 cars but all of them came out in the wrong color. The first 2 he primed with gray primer, then airbrushed it with red. It came out dark red instead of red. The 3rd one he painted w/o primer. Because the plastic was white, the red came out pinkish. What can he do so the red comes out red. It was enamels by the way. thanks.

Wonder - Welcome aboard! Glad to have you here.

My first thought on the non primed one is basically more coats of paint. It sounds as if the paint you are using is fairly thin. That's ok, I thin mine pretty well. I usually go with multiple coats, usually a minimum of three coats, sometimes more. In this case its a coverage issue, the plastic wouldn't blend and mix with the paint.
The other one ? bit more of a question. There is potential for a blending/mixing of the primer and the red. Just make sure you have let the primer dry all the way before adding the overcoat.
I would try to add another coat of two of the red (after drying) to cover the current color.
Definitely give all the other Armoramaholics a chance to chime too. Lots of good information coming from here.

I don't typically prime the rare car I build, however, I'd chose a red primer for red, brown, or similar color car. I'd use a gray primer for most other colors. As Slodder says, let the primer dry fully, especially if he's using a "hotter" paint thant the primer, that is, one whose solvent is stronger than the base color. Multiple light coats work best for a gloss finish, each done relatively soon after the previous coat. If the previous coat begins to cure too much, he should let it fully cure (at least 72 hours, maybe even more) polish it and then hit it with more light coats. As he begins to get good coverage, he may want to increase the amount of thinner and keep adding very light coats to increase the gloss. I saw an article that recommended the last coat be straight thinner. A good polishing compound, such as Carnuba Wax, or a set of dry polishing pads(up to 16,000 grit) is mandatory to get a good finish on a civilian car unless it's being done as a derelict or a beater.