For my Ork Skin method, you are going to need the following Games Workshop paints:
 
•     Dark Angels Green
 
•     Catachan Green
 
•     Goblin Green
 
•     Bleached Bone
 
•     Spare Mixing Pot (to mix the color for the final highlight)
 

STEP 1: Primer and 'Messy' Stuff

The first step is to prime the model black. I generally use Krylon Ultra Flat Black spray primer, but use whatever method works for you.

Once the primer coat is dry, take care of any 'messy work', like drybrushing, so you don't mess up your skin at a later time. As you can see here, drybrushing has the unavoidable side effect of getting paint on areas where it doesn't belong.

STEP 2: Basecoat

The next step is to give the skin areas a liberal coat of Dark Angels Green. This is to provide a green base to the skin and a dark undercoat the work up from.

You don't need to be particularly neat at this stage, so don't use your best detail brush here, save it for the really fiddly stuff. There's no need to get an even tone with this coat. It will only be visible in the darkest recesses of the finished product, so don't worry about it if there are some areas with black showing through.

STEP 3: Midtone Coat

Next, start defining the muscles and other raised areas with Catachan Green. Catachan Green has a yellowish tint to it which gives a 'warmth' to the skin tone.

When doing this step, be sure to leave some of the previous color visible in the recesses. I use a detail brush at this stage so I can make sure I 'stay within the lines' so to speak.

STEP 4: Highlight

The next step is to start defining the muscles with Goblin Green. Here's where things start to get a bit 'fiddly'. The goal here is to start giving the muscles a 'sinewy' look.

Using a very fine tip brush, paint Goblin Green onto the muscles in a 'streaking' motion, following the lines of the muscles. Be sure to leave some of the Catachan Green coat visible. As you can see from the picture, the skin is starting to get some 'depth' to it at this stage.

STEP 5: Final Highlight

The final highlight is done using a 50/50 mixture of Goblin Green and Bleached Bone. I chose Bleached Bone for the same reason that I chose Catachan Green, it has a 'warmer' look to it and makes the skin look more natural. It's a good idea to mix up a whole pot of this stuff as you'll need a lot of it when painting an ork army.

The technique in this stage is the same as in Step: 4. Paint the highlight in 'streaks', following the lines of the muscles and leaving some of the previous coat visible.

There you have it! Painting ork skin like this can be time consuming but I really like the way it looks.