It depends a bit on whether you want a real-world camouflage pattern or not. There are ways of layering shader nods, "Spots" for instance, to produce a camouflage fabric effect, but I've never heard of a good automatic way to make a realistic pattern. The pattern I created for my "Fleetmice" images is inspired by British DPM, in use for some 40 years. It's not that good a replication. But I used layers in my paint program, lightest colour as the base and the other layers not quite opaque.