I'd like to give Smudge & Morgack of the Dur-D South credit for teaching me the ways of green, and the spray bottle idea They also found that GFS has the best price and largest bottle of food coloring. They've been doing it for years. Gotta give credit where credit is due
Stell is right, Food coloring and water. You can also mix in a little sunscreen. Or put sunscreen on before you get green. Food coloring alone gives you about SPF 4. The thick skin on your hands and feet will absorb more color than the rest of your skin. I wear boots so that I don't have to dye my feet. For the face I mix less green with water in a bowl, and apply with a washcloth or paper towel. Eyebrows and the tiny hairs around your face will also absorb the green if your hair is light colored. So you might want to put lotion on them before hand. I like to paint my nails before getting green, it helps keep the green from sinking in.
Erekose taught me that rubbing chapstick into the pores on your nose helps keep the color out of them.
I use different color eyeliners and eyeshadows to bring out my inner gobbo. Black around the eyes, and black lipstick really adds to the evilness. Black hair gel to cover my humie haircolor.
Goblyns, don't forget to cover your elbows & knees ! They are goblyn errogenous zones !! Unless you're a shameless goblyn like Snotbelly.
The other weekend I tried using Ben Nye green makeup at the Utah Ren Faire. It took me 2 hours to get green !! And it rubbed off on everything, even after I set it with translucent powder. And it felt thick and icky. Still took me 2 showers and a long soak in a hot bath to get it all off. I will not be using that again.

With Ben Nye makeup
I haven't tried acrylic paints, from what I've seen they tend to peel all nasty and make you look more like a rotting zombie.
IMO, Food coloring is the way to go. A nylon bath poof and soap get the green food coloring off really well. Apricot scrub for your face. Bring a mirror into the shower with you so you can tell when your face is clean. Or a buddy

Also, Orcs might want to add a bit of black food coloring to the green ?
BTW, Crisco ? ew !
GFS Stores;
http://gfs.know-where.com/gfs/