I dont want to hijack the thread, but i figured id give my two cents on one way to make a camo, or even a multicam sling.
You would probably have to start with a light base colored sling. If yours is already camo, maybe try to bleach the exhisting colors out. And then have more of a fabric dye instead of a paint. The sling fabric would have to be as smooth as possible to prevent the different colors from bleeding in between the weave and blending into eachother.
It would depend how 'correct' you want it to look. If it's already camo, and you dye over it, then the new colors' shades would be very much effected by the colors already on the sling. If you don't mind a one of a kind custom camo, it could turn out really cool.
If that's the case, I would recommend getting some different dyes and putting them in a good water spray bottle and set it to the mist on the nozzle. or, they have fabric dyes already in spray bottles at some craft stores.
Then grab some local foliage and use that as your templates. Just base it with a light green and or tan, then lay the foliage over the sling and spray with some darker colors. When you lift the leaves off the sling, the leaf patterns will be in the original greens and tans from your base.
If you want to specifically do multicam, it would take alot more work. Probably be about the same cost as just buying a Multicam sling.
Fyi, you can buy a single linear yard of 1000 d cordura multicam on the bay for about $20 total. (A linear yard is 60 inches by 36 inches)
I've made alot of packs and pouches by just buying the stuff online because Im a newly converted, hopeless multicam junkie nowadays.
Hope that helps!