Ben Nye is really the best when it comes to appearance and being comfortable, but if you need a cheap alternative just try ordering some white/black cream makeup, the kind you use for face painting and Halloween costumes. A lot of people don't like using it because a.) it can look really streaky, b.) gets hella hot if you wear it all day, even with minimal clothes, and c.) wears off when you sweat/get wet/touch things. Also if you have bad skin it might make you break out.
Just mix the two together and apply it with your fingers or with makeup sponges, but make sure it's mixed really well and you get an even, thin coat. If you need to cover the hands or legs I'd suggest using gloves and tights (that is, if you're using cheap makeup).
If you want to take it a step further just get some eyeshadow/bronzer/blush in whatever color your Troll's blood is and apply it to the cheekbones as you would with ordinary blush, and use a darker shade along the hollows of the cheeks, the bottom of the nose, and under the chin to add some dimension to your face and not look like you're just splattered with gray paint. It takes a bit of practice but a 3 dimensional looking face is definitely going to look better than flat gray, no matter what type of makeup you're using.