Ha! I JUST had posted in an area about priming and this pretty much has set the standard of how I will prime:
Please see this site:
It is fantastic just for this graphic:
The idea is you could apply a wash OR pick a mid-tone of the primary colour you want and mist it on
with your airbrush (or buy the colour you want in a rattle-can).
The only disadvantage with all this is when you go to paint little details like armour trim and blob anything in that area, cleanup is hard to do.
I found I had to lightly airbrush over that area again with the darker colour underneath, the main colour for main surfaces and the highlight colour for the top areas like the shoulder pads on a marine.
Just like for shading: you paint the model so it is super-clean THEN apply shader: once that is on, touch-up is very hard to do well.
Some put a clear-coat on so shader flows better and is easy to clean.