Personally for effectiveness I'd recommend using free-hand so the colors all create a 'gradient' and you don't have solid colors that don't 'blend'.
The best hands down paint to use is that textured sand paint as it is already the color you want. I usually use tan, khaki, brown, and top it off with the sand texture paint. Avoid using green, black, or a lot of brown unless the field you play at is very green. Also stick with flat if possible although you may have to go with matte in some cases which is ok for airsoft.
I would recommend avoiding Krylon as I've found their paint chips easily and the nozzles clog fast. I know most people swear by them, but I've just had better luck with Rustoleum or however they spell it. Make sure you sand, wash, and prime before painting or it'll chip straight off. Also doesn't hurt to go with a clear overcoat although I question how effective the overcoat paints are. Oh, and metal likes to suck up paint but plastic tends to reject it, so do the plastic a lot more slowly or it'll 'run' and create ugly clumps of paint.
Anyway, that's the way I paint. Sorry for rambling.
