The camo rip-stop ( so-called because the grid pattern sewn into it, well... stops rips
) will be the wrong pattern unless the helmet is being sold as an Aliens helmet.You're best, if you're not getting an actual Colonial Marine helmet, to paint it up yourself at the same time as you do your armour.
You'll also need to fashion a front strike-plate for it, if you want total accuracy ( Although it does look OK without. Think the main reason for it being there is so there's room for the drop-down IR sight they used in the film, but if your head is small enough, you might be able to get away with only cutting the helmet liner... )