Both.
Some people prefer to prime their models first to get a good paint coverage and have something to bite into the plastic ... usually more with acrlyics. Others prime because they have used different mediums .. plastic, resin, brass, white metal, vinyl, rubber, and anything thats not nailed to the floor, and this means base gets a even setting.
Its quite common also to paint the base coat direct onto the kit ... if you trust it. Usually works better if its an OOB kit. I do both ... depends on what Im building and what I have added. When resin or brass is involved I always prime, then base.
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I have read about people using a darker shade of the main color of the model--say OD green with black added for a base coat, then lighter shades of OD coats on top of that.
This is what is called pre-shading. They start of with a base coat of say .. tamiya olive drab from the bottle. Paint the whole model. Then they lighten it with buff or yellow or white, etc and spray the middles of all panels and higher details ... avoiding ends and recesses. This can be lightened again and sprayed in the middle of second coating.
This gives a weathering effect like colour fading with sun rain etc, and adds depth the vehicle. really effective when done well. Then with washes/filters this can be tied together and gives a great start to weathering.
