There are lot's of ways to camo an Icon. Some I do Icon by Icon, ( or "by hand" if you prefer ) some, like vehicles that might end up being used by many nations I create maybe 8 different coloured "solid" versions then, after the Icon pallete has been applied and corrections made I simply cut one colour and paste it onto another.
I vary my technique a lot simply becasue there are so many diffent ways to do this. Here's one example that does not require a layer as you describe. It's just cut and paste. It's vital when doing it this way that both the hull and turret are positioned exactly the same for all icons in the series
Don