While I wouldn't say my skin is as good as that dudes, I do my orks by simply layering up different greens. Here's a sample:
This wasn't even that many layers. Picture is huge in the gallery if you fancy a zoom.
For this one I'm pretty sure I used the following:
Prime with black spray
Thinned chaos black wash to fill any cracks I missed
Base coat of Orkhide shade
Orkhide shade/ Snot green mix*
Devlan mud wash
Thinned snot green
Snot green and sunburst yellow mix
Thinned devlan mud wash
Snot green and lots of sunburst yellow highlight
Sunburst yellow super highlight
*Every coat from here on is a gradually smaller area than the one before. This way you get a gradient effect from the darker areas to the lighter ones.