There are a few static model camouflage techniques which might be applied to give the desired effect.  One such technique is to keep the edge of the camo paint mask slightly raised above the surface of the model, which will help produce a soft edge/feathered edge to the camo paint lines.  You can use small rolls of tape to keep the mask above the surface by  a 1/8 inch or some such (or more) to keep the edge soft.  The more space between the mask and the model surface the softer the edge will be as more paint overspray will tend to bleed under the mask.  

Normally it is wiser to only apply color where it is needed as a means to keep the weight of the paint finish to a minimum.  Hard edges between color are often "back masked" to ensure that a given color only goes on where that color is required.  This might also help to keep the level of paint/dope build up in control so as you do not end up with a "step" in the paint between colors.  Lots of pre-planning is required to do the "back mask" technique..but it will save weight in the end.

Yes..lighter color first then the darker colors. This will keep the weight down yet again as the darker colors tend to require less coats to get full coverage.

The link below shows a 1/48 scale model P-39 done up in the color scheme you have selected. It may give you some idea's about how to proceed.   Beware..the text describes the build of a F4-F Wildcat. It appear an error was made that was never corrected.  The pictures of course show the P-39.

 http://www.scaleworkshop.com/workshop/p39gh_1.htm  

edit: The following link shows the masking technique I mention above which will give up a somewhat softer edge to the camo.  This is 1/32 scale shown, but is easy to apply to large models.



...and part 2 is here:



A key piece will be having spray gear that will allow you to narrow down the spray pattern and paint flow somewhat to give the control required to produce the desire effects. The Habour Freight touch up guns will do this I believe.  

HTH.