I just wish I had Devil's skill with the airbrush - Gabby asked for the Luftwaffe style of camo blending of the grey and green.
Even though I generally use a drawing pad and stylus for this kind of cammo, you can get decent results by using the airbrush tool set to 30-40% opacity and rapidly clicking the mouse button while dragging the cursor and shaking it a little to add some randomness. Not sure I'd want to do a whole skin like this, but it's worked well enough for small areas.
The key is to build up the color slowly and don't be afraid to experiment with the brush widths to achieve the level of overspray you're trying to mimic. There isn't a universal setting that works in all cases - hell, you might need to use different settings on different parts of the same skins due to differences in the scaling.
Also, I had this image of Gabby's Jug kicking around on my machine. Looks like a legit color photo to me.