What says the forum: Paint and primer in one?

[rgG]

The primer in the paint and primer just helps the paint adhere better.
It is NOT a good substitute for a real primer on bare wood.
Give them a coat of real primer and top coat with whatever paint you want.
If the wood is pine or a wood that has knots that might bleed sap, use a shellac based primer.
Since you will be going with a dark top coat, get the store to tint the primer a little towards the final cover, to make coverage easier.
If it is a really dark green, plan on a tinted prime coat, then two coats of the top coat.
Since this is a exterior project, a proper primer is even more important.

Edit: if you prefer to use spray, there are also spray primers, then spray the top coats.