flores, when painting cammo, yes, lighter colors first and work your way towards the darker ones. we share the same addiction as i love the cammo shceme myself and when i go model hunting, i buy atleast one bird with a cammo scheme every trip. ironically, the P-51D is my all time favorite bird and it is usually found in natural metal. when doing cammo schemes freehand i have found these methods to work well for me.
1. you need a good quality airbrush and the ability to achieve the fine line consistantly is key for freehand jobs, masking is a whole different ball game that i dont have the patience for. a steady hand is neccesary as well. i use an iwata hp-c or hp-cs but there are plenty more good brushes out there as well. BTW, it needs to be clean too!! along with the airbrush you need some type of air source, moisture trap, and pressure regulator.
2. if you have the above in place and all sanding, filling, and cleaning are done, the next step is to mix the paint. let me harp on a clean, flawless surface for just a second, the paint job is only gonna look as good as the surface, so do what you have to do (priming to find defects and wet sanding them out ect, ect) to get them out before you begin the paint job. for the base coat, i usually mix a 60% paint to 40% thinner ratio and mist the base coat on in light even coats using around 15 psi. this usually takes about 3 coats to get the results i'm looking for. when i get the base coat done and it has cured over night ( i use mm enamels ), i take a pencil and lightly trace the edges of the next color onto the surface of the model. then, thinning a little further i take the next lightest color with a 50/50 ratio and begin to follow the pencil marks. at this point, i'm not concerning myself with the tight line on the edges because its gonna take a couple of coats to get the next color to cover as well. just get the general shape of the cammo pattern filled in and get the coverage you want. do the same with the next color and the next if you have that many. i usually apply these color with about 10 to 12 psi. usually, the thinner the paint, the lower the pressure needs to be.
3. at this point, you have all the colors applied to the model and your very excited because it sorta resembles the real bird and you want to decal it and finish it and show it to your forum friends!!! NOT!!! its gonna look horrible as the edges look all grainy and you messed up over here and had an uh-oh over there. so, take another thinner ratio of each color ( about 40% paint and 60% thinner at about 8 psi or so) and begin the long, slow journey (but boy this is my favorite part!!!) of cleaning up the edges. now, while doing this, your gonna be into all of the colors 2 or 3 times erasing little boo boos here and there. this is where good old fashion patience and a desire to get it just right come into place. well, these are the methods that i use and below are some pics to give you an idea of what my humble efforts achieve. hope this helps. later.