Hi. Okay, my washing experience is, to say the least, not very extensive, but I'll pass this on from my experience. I 've had real good luck with Raw Umber or Raw Umber and a little Mars black. Or you maybe some raw umber and burnt sienna. Depends on how dark or light you want it to be. I just use this very sparingly to let it flow down into all the little cracks and crevices and such.
As far as drybrushing goes, since you have the 3-color camo, I'd say a dryburhsing with lightened colors of the three basic paints you've used, then a little 'dust' ,grime, etc. and the usual things like that.
Experiment on some other stuff to see if this is what you're going for on your 'kitty'.
Take care, Sgirty