Primer?

I just finished building the Tamiya 1/16 scale King Tiger, and recommend a primer coat for most parts. Primer adds "tooth" to the parts, which allows the paint to adhere more firmly to the parts. Also, sometimes a coat of primer makes surface defects that you've missed more visible.
I usually give a model a light sand with 600-800 to get rid of mold parting lines and to "roughen" up the surface a little so the paint will have more "tooth" or surface area to cling to - it makes the paint stick better and can prevent paint flaking off a model.
Use as light a colored primer as you can for things like cars, as (for example) putting red top coat (on a car) over dark grey primer would need about a thousand coats of red to get a full deep color; whereas if you primed the car with flat white, you'd only need the "normal" number of coats to get a rich deep red.
For the King Kiger, I used a mid gray primer, then flat black was sprayed along all the panel lines and recesses. When I started with the color coats, I thinned the paint a bit and now the panel lines kind of stand out because they have a darker color on them. Get it?
You certainly don't need to prime for EACH new color, just the once.
I use a lot of different paints to prime - it just depends on what kind of paint I plan on using on the model.
I use automotive acrylic on some things like airliners and some cars- it dries fast and sands and polishes within a few hours of application. Apply sparingly as the thinner in the paint is "hot" and etches styrene, too much can warp your model (I'm not kidding).
Model Master makes great enamel paint - they've got a lot of greys you can use, and Tamiya acrylic is also very good.
As a base rule, until you gain experience in what damaage one type of paint can do to another (enamel over acrylic for example) it's best to use the same type of paint for priming and topcoats.
When in doubt, EXPERIMENT. It's easier and cheaper than finding out too late that a type of paint eats another.
Have fun!!
Lee Tree

I use Testors light gray enamel primer on most of my projects. I work almost exclusively in armor, (armor is very forgiving of mistakes). The primer shows any flaws in your work before you start with the base coat. I've had real good luck with this primer under both enamel and acrylic paints.

My thoughts are that you generally need primer for two reasons:
1. To provide an even surface that covers fillers and other imperfections in the underlying plastic/surface. May not seem like much, but you'll kick yourself the next time you're applying a color and that one 'spot' seems to keep showing through the paint.
2. To give the paint something to adhere to. Many paints will stick well to plastic, but some do not. Most (if not all) paints will stick very well to primer. Again, you'll kick yourself when you're almost done and you find the paint 'flaking' off in some areas.

I don't use it 100% of the time, but definitely use it where I want even coverage or where I expect the model will be handled or masked.

M.