I was able to duplicate that paint effect.
I took a crank that I'd already painted and topcoated, and sprayed it with a mixture of my water/dishwashing liquid cleaner and opaque black Createx. I used an already top coated lure because I wanted the gloss finish so the black mixture wouldn't bond really well to the crank.
I filled my airbrush cup half full of the cleaner, added three drops of the black, mixed it in the cup, and then sprayed it down the back of the crank from 18" away with light pressure. I held the painted crank still for a minute, and then hung it to air dry. Because the paint is so diluted, it has no film strength, so it's really fragile until it dries.
I got the same paint effect.
I thought about it last night, and I realized that it looked like the paint hadn't mixed with the thinning agent, but had clumped together like an emulsion. Liquid dish washing soap is an emulsifier, breaking up grease into tiny globules that can be washed away with hot water. I think I had added my original 4011 to my airbrush cup with some of the cleaning fluid still in it the first time it happened. Kind of a happy accident.
Now that I know how to do it, I'll paint my cranks up to the mottled black back, shoot a coat of gloss clear over them, and then come back the next day and spray my water/soap/black paint mix over the back. Once that air dries, so my hair dryer doesn't move the black around, I'll shoot two more coats of clear over them, and I'll be done. And I'll try it with other paint schemes, too. I bet it would look good on the back and sides of a black and red craw.
I'll take a picture tomorrow, once it's dried, and post it here.