The only paints you should ever use on toys are waterbased... Basically, if you clean up using water (Not paint thinner) then you're good... I'd suggest Tamiya paint if you have them available in your area...
Now, for painting camo, especially on large surfaces like a vehicle, you have a dilemma... Doing it by hand is going to Su-diddly-uck... You're better off trying spray paint then, or using an airbrush. Since you're not a big customizer I'm guessing the airbrush is out... Spray paint would work, so long as you find one that won't react with the plastic.
I'd maybe try buying a small can of Testors spray, and just spray it on a VERY small corner or piece of each piece of the AT-AT just so you see if it'll react poorly... It may not, and then you're in business.
As for colors, you should just compare and see what you like... Then ask yourself if you want woodland, or desert, or winter, or autumn, urban, night-time, etc... Camo patterns are vast and varied so you may not want to go with the basic U.S. army woodland, though it's easier than most IMO.
Then, you'd prime the piece... Or I would... Well, first you s hould try cleaning and drying the AT-AT to get all dirt, oil, etc., off of it... Then prime coat it. Then mask the pattern you want.... Paint the first color, let dry, then mask off the areas not to be coated the next color and paint... Let dry and do the 3rd color, etc...
Camo can be a huge pain, haha...
As you can see. Especially on a bigger item.
