This is a method I found on the web when looking at 1:1 painted rifles and works quite well in 1:6 I think.
You will need
Spray paint or air brush
Fake grass from the Rail Road section of your local hobby shop
Fine grit sand paper
My Para rescue figure needs a new paint job from his M14.
First you want to break down the rifle and tape off any thing you do not want to get painted. Paint a base color over the whole rifle. I am using Model Masters Marine Tan for my base coat.
After it dries put the fake grass on top of the rifle as shown below and tape the grass in place above and below the rifle.
Then paint your first camo color. I use short pules of paint with the minimum distance between the air brush and the rifle is 12 inches. I shoot 5 pules along the rifle from directly above it and then 5 more at 45 degrees above and below the rifle. I am using Model Masters Dark Tan. You can just see it on the stock and other places as it will darken as it dries.
I then repeat the process with as many colors as you want, waiting at least 15 min between each paint application. I did it with Model Masters OD Green and this is what it will look like after you finish.
Now weather with sand paper or dry brush what ever is more appropriate after the paint dries. Here is my SAS sniper rifle and my ACE M14. The M14 was done with the same colors as the other M14 so you can get an idea of what it will look like when dry.