In 1/144 I would recommend airbrushing the upper base colour in enamel (sand colour in the case of an IDF A4) followed with careful and even masked airbrushing the lower colour in enamel - this gives a primed surface that will take acrylic paint nicely.. and then apply the cam by hand brushing in suitably thinned acrylic. Acrylic won't mix with the enamel. The real danger with hand brushing the cam element with enamel is the chance of the colours mixing where you don't want them to.
1/144 can be a nightmare with masking a cam for airbrushing and freehand airbrushing will result in too much overspray here and there.
There's not much of a feathered edge to see in 1/144 so hand brushing will work fine for demarcation of green/brown over sand and you'll hopefully achieve the desired effect on the first pass !
Use a fine brush - take your time - and if the colours look too stark (as they may the smaller in scale one goes) an overspray of the whole aircraft with a matt coat that has a touch of grey in it (I use Testors dullcote 95% / Humbrol lt grey 5%) will bring it down to scale and will soften the demarcations... but I recommend trying this technique out on a test subject first !
Here's one we prepared earlier using the technique I described above.
HTH
