Since I've recieved a couple of comments regarding this, a disclaimer:
This is NOT a tutorial for the classic "blob" camouflage. That horse is dead and bludgeoned. If you want the patchy camo used by modern militaries, I'm sure there are other people who could do a better job than I. This is for active camo, that is, camouflage that is constantly shifting to match the surroundings. The cloak is meant to portray a shifting kaleidoscope of woodland colors, not a static pattern.
Now that that's out of the way, on to the tutorial.
What You'll Need:
One medium drybrush
One or more primed models with camo cloaks
Several shades of green paint, at least one brown, and a dark green wash. As the picture will show (if the resolution is good enough), I used Knarloc Green, Catachan Green, Snot Green, and Orkhide Shade for my greens, Vermin Brown for the brown, and Thraka Green for the wash.
Before the Camo Job, a plain, boring black cloak.
Step One: Dip the tip of the brush in your first green (Knarloc Green). Don't get too much on the brush. You want a thin coat. Paint in short strokes, and try to get the paint to cover most of the cloak.
Step Two: Take your next shade (Catachan Green) and again, not too much on the brush. Wipe some off if you have to. You'll want those same short strokes. With this color, try to fill in some of the gaps in the paint job. I found that it really helps create that "shifting pattern" look if you stroke in one direction with each coat and vary the direction of your strokes with each shade.
Step Three: By now most of the cloak should be covered, and you know the drill. Next color, little bit of paint, short brush strokes, aim for the unpainted spots.
Step Four: The final green should be used to make sure any remaining unpainted areas get some love.
Step Five: So now you've got a cloak with four mottled shades of green, which is great as far as it goes, but can feel a little boring, so I spiced it up with a little bit of Vermin Brown to simulate branches and whatnot.
Make sure you get the top, over the shoulders, and if you want, the front of the cloak as well (the side facing the model).
Step Six: You're almost done! Now just take that nice, dark green wash, and give a generous coat to the whole cloak. Not only does the green wash serve to smooth the transition between shades, it also provides the sense that the cloak is shifting colors rather than one unchanging pattern.
And there you have it. Chameleon cloaks for Rangers, Scouts, Catachans, Chaos cultists, camo netting for vehicles and HW teams, or anything else that needs to be ded sneaky. If you're feeling adventurous, you could try something similar with various whites and greys for the snow-and-ice look, browns and tans for desert camo, or black-and-red mottling for volcanic worlds. If I find myself painting more cloaked models, I might try one of those ideas myself and post a follow-up.
These would probably look absolutely stunning with some hooded heads, like those from the Dark angels upgrade sprue.
In closing, I leave you with Scout Squad Thanatos!