I have recently succumbed to painting Playmobil cars and have used the Halfords stock paint and Halfords plastic primer with success.
I recently asked the guys how easy it was to get non-stock paint colours mixed and they said it was easy, but they thought I would need the paint code rather than just telling them I wanted Ford Focus RS apple green for example.
The best advice I can give you (incase you haven't already done it) is take the truck apart into it's component parts as it makes it easier to paint. One thing to watch for though is that when you put it back together again it is very tight as you have added coats of paint to something that fitted together perfectly already.
Also, make sure you spray in a very well ventilated area as the paint, and especially the primer will go everywhere!!
Oh and one other thing....painting Playmobil is addictive!!
Claire