well, if you're painting a camo pattern, I find the best way is to lightly draw your pattern on the plane with a pencil. Don't push too hard or you'll end up leaving a depression in the primer which will show through with your overcoat.
There are two ways you can go; 1) start with primer as your base coat and paint on top of that, or 2) on top of the primer, lay down a base coat of light tan, draw your pattern on top of that and leave blank spaces for the tan to show through. It's one less color you have to worry about.
Once you've layed out your pattern in pencil, come up with a color code (i.e. 1=green, 2=brown, etc.) and then go back and number random patches with the code. Now you don't have to think about it. Just make sure that if you're going to have a tan base coat, not to number those blank areas.
Hope it helps,
Fred