That sort of depends on two things.
One is if your paint sticks really well to the plastic when masked, so you don't need any primer coat.
The other is how thick you put your paint layers on. If you are painting really thin, then each coat of paint you put on with have "colored preshading" at the lines where the colors overlap. You can do this on purpose, to get a good "blending in" effect, if you paint the first and each succeeding color exactly where you want the separation line to be (sort of how it is done in real life)
if you need a primer, or if you think you will have color bleed through,,,,,,,then you should put the light color over the whole airframe, then the next color, then the last.
It really depends on your own painting style.
Rex (former Cheesehead from Wis Dells)